diff --git a/java/src/org/broadinstitute/sting/playground/gatk/walkers/annotator/GenomicAnnotator.java b/java/src/org/broadinstitute/sting/playground/gatk/walkers/annotator/GenomicAnnotator.java
index cf08e9b58..914c356c7 100644
--- a/java/src/org/broadinstitute/sting/playground/gatk/walkers/annotator/GenomicAnnotator.java
+++ b/java/src/org/broadinstitute/sting/playground/gatk/walkers/annotator/GenomicAnnotator.java
@@ -56,6 +56,7 @@ import org.broadinstitute.sting.gatk.refdata.features.annotator.AnnotatorInputTa
 import org.broadinstitute.sting.gatk.walkers.By;
 import org.broadinstitute.sting.gatk.walkers.DataSource;
 import org.broadinstitute.sting.gatk.walkers.RodWalker;
+import org.broadinstitute.sting.gatk.walkers.TreeReducible;
 import org.broadinstitute.sting.gatk.walkers.annotator.VariantAnnotatorEngine;
 import org.broadinstitute.sting.utils.BaseUtils;
 import org.broadinstitute.sting.utils.SampleUtils;
@@ -73,7 +74,7 @@ import org.broadinstitute.sting.utils.genotype.vcf.VCFWriter;
 //@Allows(value={DataSource.READS, DataSource.REFERENCE})
 //@Reference(window=@Window(start=-50,stop=50))
 @By(DataSource.REFERENCE)
-public class GenomicAnnotator extends RodWalker<Integer, Integer> {
+public class GenomicAnnotator extends RodWalker<LinkedList<VCFRecord>, LinkedList<VCFRecord>> implements TreeReducible<LinkedList<VCFRecord>> {
     @Argument(fullName="vcfOutput", shortName="vcf", doc="VCF file to which all variants should be written with annotations", required=true)
     protected File VCF_OUT;
 
@@ -96,19 +97,20 @@ public class GenomicAnnotator extends RodWalker<Integer, Integer> {
 
     private boolean strict = true;
 
+    private boolean multiThreadedMode = false; //whether map will be called by more than one thread.
 
     /**
      * Prepare the output file and the list of available features.
      */
     public void initialize() {
 
+        multiThreadedMode = getToolkit().getArguments().numberOfThreads > 1;
+
         // get the list of all sample names from the various VCF input rods
         TreeSet<String> samples = new TreeSet<String>();
         SampleUtils.getUniquifiedSamplesFromRods(getToolkit(), samples, new HashMap<Pair<String, String>, String>());
 
 
-
-
         //read all ROD file headers and construct a set of all column names to be used for validation of command-line args
         final Set<String> allFullyQualifiedColumnNames = new LinkedHashSet<String>();
         final Set<String> allBindingNames = new LinkedHashSet<String>();
@@ -247,7 +249,7 @@ public class GenomicAnnotator extends RodWalker<Integer, Integer> {
      *
      * @return 0
      */
-    public Integer reduceInit() { return 0; }
+    public LinkedList<VCFRecord> reduceInit() { return new LinkedList<VCFRecord>(); }
 
 
     /**
@@ -265,23 +267,25 @@ public class GenomicAnnotator extends RodWalker<Integer, Integer> {
      * @param context  the context for the given locus
      * @return 1 if the locus was successfully processed, 0 if otherwise
      */
-    public Integer map(RefMetaDataTracker tracker, ReferenceContext ref, AlignmentContext context) {
+    public LinkedList<VCFRecord> map(RefMetaDataTracker tracker, ReferenceContext ref, AlignmentContext context) {
+        LinkedList<VCFRecord> result = new LinkedList<VCFRecord>();
+
         if ( tracker == null )
-            return 0;
+            return result;
 
         List<Object> rods = tracker.getReferenceMetaData("variant");
         // ignore places where we don't have a variant
         if ( rods.size() == 0 )
-            return 0;
+            return result;
 
         Object variant = rods.get(0);
         if( BaseUtils.isNBase(ref.getBase()) ) {
-            return 0; //TODO Currently, VariantContextAdaptors.toVCF(annotatedVC, ref.getBase()) fails when base is 'N'. is this right?
+            return result; //TODO Currently, VariantContextAdaptors.toVCF(annotatedVC, ref.getBase()) fails when base is 'N'. is this right?
         }
 
         VariantContext vc = VariantContextAdaptors.toVariantContext("variant", variant, ref);
         if ( vc == null )
-            return 0;
+            return result;
 
         // if the reference base is not ambiguous, we can annotate
         Collection<VariantContext> annotatedVCs = Arrays.asList(vc);
@@ -292,40 +296,69 @@ public class GenomicAnnotator extends RodWalker<Integer, Integer> {
             }
         }
 
-        if ( variant instanceof VCFRecord) { //TODO is this if-statement necessary?
+        if(multiThreadedMode) {
+            //keep results in memory, only writing them in onTraversalDone(..) after they have been merged via treeReduce(..)
+            for(VariantContext annotatedVC : annotatedVCs ) {
+                result.add(VariantContextAdaptors.toVCF(annotatedVC, ref.getBase()));
+            }
+        } else {
+            //write results to disk immediately
             for(VariantContext annotatedVC : annotatedVCs ) {
                 vcfWriter.addRecord(VariantContextAdaptors.toVCF(annotatedVC, ref.getBase()));
             }
         }
 
-        return annotatedVCs.size();
+
+        return result;
     }
 
 
     /**
-     * Increment the number of loci processed.
+     * Merge lists.
      *
      * @param value result of the map.
      * @param sum   accumulator for the reduce.
      * @return the new number of loci processed.
      */
-    public Integer reduce(Integer value, Integer sum) {
-        return sum + value;
+    public LinkedList<VCFRecord> reduce(LinkedList<VCFRecord> value, LinkedList<VCFRecord> sum) {
+        sum.addAll(value);
+        return sum;
     }
 
+
+
+    /**
+     * Merge lists.
+     */
+    public LinkedList<VCFRecord> treeReduce(LinkedList<VCFRecord> lhs, LinkedList<VCFRecord> rhs) {
+        lhs.addAll(rhs);
+        return lhs;
+    }
+
+
+
+
     /**
      * Tell the user the number of loci processed and close out the new variants file.
      *
      * @param result  the number of loci seen.
      */
-    public void onTraversalDone(Integer totalOutputVCFRecords) {
-        out.printf("Generated %d annotated VCF records.\n", totalOutputVCFRecords);
+    public void onTraversalDone(LinkedList<VCFRecord> totalOutputVCFRecords) {
+        if(multiThreadedMode) {
+            //finally write results to disk
+            for(VCFRecord vcfRecord : totalOutputVCFRecords ) {
+                vcfWriter.addRecord(vcfRecord);
+            }
+        }
+
+        //out.printf("Generated %d annotated VCF records.\n", totalOutputVCFRecords);
         Map<String, Integer> inputTableHitCounter = engine.getInputTableHitCounter();
         for(Entry<String, Integer> e : inputTableHitCounter.entrySet()) {
             final String bindingName = e.getKey();
             final int counter = e.getValue();
-            final float percent = 100 * counter /(float) totalOutputVCFRecords;
-            out.printf(" %-6.1f%%   (%d) annotated with %s.\n", percent, counter, bindingName );
+            //final float percent = 100 * counter /(float) totalOutputVCFRecords;
+            //out.printf(" %-6.1f%%   (%d) annotated with %s.\n", percent, counter, bindingName );
+            out.printf(" %d annotated with %s.\n", counter, bindingName );
         }
 
 
@@ -333,6 +366,5 @@ public class GenomicAnnotator extends RodWalker<Integer, Integer> {
     }
 
 
-
 }