diff --git a/solr/CHANGES.txt b/solr/CHANGES.txt
index d1275cf0f39..8a3ad22e8e1 100644
--- a/solr/CHANGES.txt
+++ b/solr/CHANGES.txt
@@ -429,6 +429,8 @@ New Features
 * SOLR-2263: Add ability for RawResponseWriter to stream binary files as well as
   text files.  (Eric Pugh via yonik)
 
+* SOLR-860: Add debug output for MoreLikeThis. (koji)
+
 Optimizations
 ----------------------
 
diff --git a/solr/src/java/org/apache/solr/handler/MoreLikeThisHandler.java b/solr/src/java/org/apache/solr/handler/MoreLikeThisHandler.java
index e367d8922f4..fcd41e24dd9 100644
--- a/solr/src/java/org/apache/solr/handler/MoreLikeThisHandler.java
+++ b/solr/src/java/org/apache/solr/handler/MoreLikeThisHandler.java
@@ -232,7 +232,7 @@ public class MoreLikeThisHandler extends RequestHandlerBase
     // Copied from StandardRequestHandler... perhaps it should be added to doStandardDebug?
     if (dbg == true) {
       try {
-        NamedList<Object> dbgInfo = SolrPluginUtils.doStandardDebug(req, q, mlt.mltquery, mltDocs.docList, dbgQuery, dbgResults);
+        NamedList<Object> dbgInfo = SolrPluginUtils.doStandardDebug(req, q, mlt.getRawMLTQuery(), mltDocs.docList, dbgQuery, dbgResults);
         if (null != dbgInfo) {
           if (null != filters) {
             dbgInfo.add("filter_queries",req.getParams().getParams(CommonParams.FQ));
@@ -279,8 +279,6 @@ public class MoreLikeThisHandler extends RequestHandlerBase
     final boolean needDocSet;
     Map<String,Float> boostFields;
     
-    Query mltquery;  // expose this for debugging
-    
     public MoreLikeThisHelper( SolrParams params, SolrIndexSearcher searcher )
     {
       this.searcher = searcher;
@@ -310,9 +308,26 @@ public class MoreLikeThisHandler extends RequestHandlerBase
       boostFields = SolrPluginUtils.parseFieldBoosts(params.getParams(MoreLikeThisParams.QF));
     }
     
-    private void setBoosts(Query mltquery) {
+    private Query rawMLTQuery;
+    private Query boostedMLTQuery;
+    private BooleanQuery realMLTQuery;
+    
+    public Query getRawMLTQuery(){
+      return rawMLTQuery;
+    }
+    
+    public Query getBoostedMLTQuery(){
+      return boostedMLTQuery;
+    }
+    
+    public Query getRealMLTQuery(){
+      return realMLTQuery;
+    }
+    
+    private Query getBoostedQuery(Query mltquery) {
+      BooleanQuery boostedQuery = (BooleanQuery)mltquery.clone();
       if (boostFields.size() > 0) {
-        List clauses = ((BooleanQuery)mltquery).clauses();
+        List clauses = boostedQuery.clauses();
         for( Object o : clauses ) {
           TermQuery q = (TermQuery)((BooleanClause)o).getQuery();
           Float b = this.boostFields.get(q.getTerm().field());
@@ -321,49 +336,51 @@ public class MoreLikeThisHandler extends RequestHandlerBase
           }
         }
       }
+      return boostedQuery;
     }
     
     public DocListAndSet getMoreLikeThis( int id, int start, int rows, List<Query> filters, List<InterestingTerm> terms, int flags ) throws IOException
     {
       Document doc = reader.document(id);
-      mltquery = mlt.like(id);
-      setBoosts(mltquery);
+      rawMLTQuery = mlt.like(id);
+      boostedMLTQuery = getBoostedQuery( rawMLTQuery );
       if( terms != null ) {
-        fillInterestingTermsFromMLTQuery( mltquery, terms );
+        fillInterestingTermsFromMLTQuery( rawMLTQuery, terms );
       }
 
       // exclude current document from results
-      BooleanQuery mltQuery = new BooleanQuery();
-      mltQuery.add(mltquery, BooleanClause.Occur.MUST);
-      mltQuery.add(
+      realMLTQuery = new BooleanQuery();
+      realMLTQuery.add(boostedMLTQuery, BooleanClause.Occur.MUST);
+      realMLTQuery.add(
           new TermQuery(new Term(uniqueKeyField.getName(), uniqueKeyField.getType().storedToIndexed(doc.getFieldable(uniqueKeyField.getName())))), 
             BooleanClause.Occur.MUST_NOT);
       
       DocListAndSet results = new DocListAndSet();
       if (this.needDocSet) {
-        results = searcher.getDocListAndSet(mltQuery, filters, null, start, rows, flags);
+        results = searcher.getDocListAndSet(realMLTQuery, filters, null, start, rows, flags);
       } else {
-        results.docList = searcher.getDocList(mltQuery, filters, null, start, rows, flags);
+        results.docList = searcher.getDocList(realMLTQuery, filters, null, start, rows, flags);
       }
       return results;
     }
 
     public DocListAndSet getMoreLikeThis( Reader reader, int start, int rows, List<Query> filters, List<InterestingTerm> terms, int flags ) throws IOException
     {
-      mltquery = mlt.like(reader);
-      setBoosts(mltquery);
+      rawMLTQuery = mlt.like(reader);
+      boostedMLTQuery = getBoostedQuery( rawMLTQuery );
       if( terms != null ) {
-        fillInterestingTermsFromMLTQuery( mltquery, terms );
+        fillInterestingTermsFromMLTQuery( boostedMLTQuery, terms );
       }
       DocListAndSet results = new DocListAndSet();
       if (this.needDocSet) {
-        results = searcher.getDocListAndSet(mltquery, filters, null, start, rows, flags);
+        results = searcher.getDocListAndSet( boostedMLTQuery, filters, null, start, rows, flags);
       } else {
-        results.docList = searcher.getDocList(mltquery, filters, null, start, rows, flags);
+        results.docList = searcher.getDocList( boostedMLTQuery, filters, null, start, rows, flags);
       }
       return results;
     }
-    
+
+    @Deprecated
     public NamedList<DocList> getMoreLikeThese( DocList docs, int rows, int flags ) throws IOException
     {
       IndexSchema schema = searcher.getSchema();
@@ -382,7 +399,7 @@ public class MoreLikeThisHandler extends RequestHandlerBase
     
     private void fillInterestingTermsFromMLTQuery( Query query, List<InterestingTerm> terms )
     { 
-      List clauses = ((BooleanQuery)mltquery).clauses();
+      List clauses = ((BooleanQuery)query).clauses();
       for( Object o : clauses ) {
         TermQuery q = (TermQuery)((BooleanClause)o).getQuery();
         InterestingTerm it = new InterestingTerm();
diff --git a/solr/src/java/org/apache/solr/handler/component/MoreLikeThisComponent.java b/solr/src/java/org/apache/solr/handler/component/MoreLikeThisComponent.java
index 61c97d1faf4..8851ff7761d 100644
--- a/solr/src/java/org/apache/solr/handler/component/MoreLikeThisComponent.java
+++ b/solr/src/java/org/apache/solr/handler/component/MoreLikeThisComponent.java
@@ -23,8 +23,12 @@ import java.net.URL;
 import org.apache.solr.common.params.MoreLikeThisParams;
 import org.apache.solr.common.params.SolrParams;
 import org.apache.solr.common.util.NamedList;
+import org.apache.solr.common.util.SimpleOrderedMap;
 import org.apache.solr.handler.MoreLikeThisHandler;
+import org.apache.solr.schema.IndexSchema;
+import org.apache.solr.search.DocIterator;
 import org.apache.solr.search.DocList;
+import org.apache.solr.search.DocListAndSet;
 import org.apache.solr.search.SolrIndexSearcher;
 
 /**
@@ -50,18 +54,59 @@ public class MoreLikeThisComponent extends SearchComponent
     if( p.getBool( MoreLikeThisParams.MLT, false ) ) {
       SolrIndexSearcher searcher = rb.req.getSearcher();
       
-      MoreLikeThisHandler.MoreLikeThisHelper mlt 
-        = new MoreLikeThisHandler.MoreLikeThisHelper( p, searcher );
-      
-      int mltcount = p.getInt( MoreLikeThisParams.DOC_COUNT, 5 );
-      NamedList<DocList> sim = mlt.getMoreLikeThese(
-          rb.getResults().docList, mltcount, rb.getFieldFlags() );
+      NamedList<DocList> sim = getMoreLikeThese( rb, searcher,
+          rb.getResults().docList, rb.getFieldFlags() );
 
       // TODO ???? add this directly to the response?
       rb.rsp.add( "moreLikeThis", sim );
     }
   }
 
+  NamedList<DocList> getMoreLikeThese( ResponseBuilder rb, SolrIndexSearcher searcher,
+      DocList docs, int flags ) throws IOException {
+    SolrParams p = rb.req.getParams();
+    IndexSchema schema = searcher.getSchema();
+    MoreLikeThisHandler.MoreLikeThisHelper mltHelper 
+      = new MoreLikeThisHandler.MoreLikeThisHelper( p, searcher );
+    NamedList<DocList> mlt = new SimpleOrderedMap<DocList>();
+    DocIterator iterator = docs.iterator();
+
+    SimpleOrderedMap<Object> dbg = null;
+    if( rb.isDebug() ){
+      dbg = new SimpleOrderedMap<Object>();
+    }
+
+    while( iterator.hasNext() ) {
+      int id = iterator.nextDoc();
+      int rows = p.getInt( MoreLikeThisParams.DOC_COUNT, 5 );
+      DocListAndSet sim = mltHelper.getMoreLikeThis( id, 0, rows, null, null, flags );
+      String name = schema.printableUniqueKey( searcher.doc( id ) );
+      mlt.add(name, sim.docList);
+      
+      if( dbg != null ){
+        SimpleOrderedMap<Object> docDbg = new SimpleOrderedMap<Object>();
+        docDbg.add( "rawMLTQuery", mltHelper.getRawMLTQuery().toString() );
+        docDbg.add( "boostedMLTQuery", mltHelper.getBoostedMLTQuery().toString() );
+        docDbg.add( "realMLTQuery", mltHelper.getRealMLTQuery().toString() );
+        SimpleOrderedMap<Object> explains = new SimpleOrderedMap<Object>();
+        DocIterator mltIte = sim.docList.iterator();
+        while( mltIte.hasNext() ){
+          int mltid = mltIte.nextDoc();
+          String key = schema.printableUniqueKey( searcher.doc( mltid ) );
+          explains.add( key, searcher.explain( mltHelper.getRealMLTQuery(), mltid ) );
+        }
+        docDbg.add( "explain", explains );
+        dbg.add( name, docDbg );
+      }
+    }
+
+    // add debug information
+    if( dbg != null ){
+      rb.addDebugInfo( "moreLikeThis", dbg );
+    }
+    return mlt;
+  }
+  
   /////////////////////////////////////////////
   ///  SolrInfoMBean
   ////////////////////////////////////////////
diff --git a/solr/src/test/org/apache/solr/handler/MoreLikeThisHandlerTest.java b/solr/src/test/org/apache/solr/handler/MoreLikeThisHandlerTest.java
index 63b1edde582..6dbae21f244 100644
--- a/solr/src/test/org/apache/solr/handler/MoreLikeThisHandlerTest.java
+++ b/solr/src/test/org/apache/solr/handler/MoreLikeThisHandlerTest.java
@@ -94,7 +94,17 @@ public class MoreLikeThisHandlerTest extends SolrTestCaseJ4 {
     assertQ("morelike this - harrison ford",mltreq
         ,"//result/doc[1]/int[@name='id'][.='45']");
 
+    // test MoreLikeThis debug
+    params.set(CommonParams.DEBUG_QUERY, "true");
+    assertQ("morelike this - harrison ford",mltreq
+        ,"//lst[@name='debug']/lst[@name='moreLikeThis']/lst[@name='44']/str[@name='rawMLTQuery']"
+        ,"//lst[@name='debug']/lst[@name='moreLikeThis']/lst[@name='44']/str[@name='boostedMLTQuery']"
+        ,"//lst[@name='debug']/lst[@name='moreLikeThis']/lst[@name='44']/str[@name='realMLTQuery']"
+        ,"//lst[@name='debug']/lst[@name='moreLikeThis']/lst[@name='44']/lst[@name='explain']/str[@name='45']"
+        );
+
     // test that qparser plugins work
+    params.remove(CommonParams.DEBUG_QUERY);
     params.set(CommonParams.Q, "{!field f=id}44");
     assertQ(mltreq
         ,"//result/doc[1]/int[@name='id'][.='45']");
@@ -112,9 +122,9 @@ public class MoreLikeThisHandlerTest extends SolrTestCaseJ4 {
     assertQ(mltreq
         ,"//result/doc[1]/int[@name='id'][.='45']");
 
-    // test that debugging works
+    // test that debugging works (test for MoreLikeThis*Handler*)
     params.set(CommonParams.QT, "/mlt");
-    params.set("debugQuery", "true");
+    params.set(CommonParams.DEBUG_QUERY, "true");
     assertQ(mltreq
         ,"//result/doc[1]/int[@name='id'][.='45']"
         ,"//lst[@name='debug']/lst[@name='explain']"