diff --git a/public/java/src/org/broadinstitute/sting/utils/codecs/vcf/SortingVCFWriterBase.java b/public/java/src/org/broadinstitute/sting/utils/codecs/vcf/SortingVCFWriterBase.java
index c299511db..84ecc7fcd 100755
--- a/public/java/src/org/broadinstitute/sting/utils/codecs/vcf/SortingVCFWriterBase.java
+++ b/public/java/src/org/broadinstitute/sting/utils/codecs/vcf/SortingVCFWriterBase.java
@@ -27,10 +27,8 @@ package org.broadinstitute.sting.utils.codecs.vcf;
 
 import org.broadinstitute.sting.utils.variantcontext.VariantContext;
 
-import java.util.Comparator;
-import java.util.PriorityQueue;
-import java.util.Set;
-import java.util.TreeSet;
+import java.util.*;
+import java.util.concurrent.PriorityBlockingQueue;
 
 /**
  * This class writes VCF files, allowing records to be passed in unsorted.
@@ -39,20 +37,26 @@ import java.util.TreeSet;
 public abstract class SortingVCFWriterBase implements VCFWriter {
 
     // The VCFWriter to which to actually write the sorted VCF records
-    private VCFWriter innerWriter = null;
+    private final VCFWriter innerWriter;
 
     // the current queue of un-emitted records
-    private PriorityQueue<VCFRecord> queue = null;
+    private final Queue<VCFRecord> queue;
 
     // The locus until which we are permitted to write out (inclusive)
     protected Integer mostUpstreamWritableLoc;
     protected static final int BEFORE_MOST_UPSTREAM_LOC = 0; // No real locus index is <= 0
 
     // The set of chromosomes already passed over and to which it is forbidden to return
-    private Set<String> finishedChromosomes = null;
+    private final Set<String> finishedChromosomes;
 
     // Should we call innerWriter.close() in close()
-    private boolean takeOwnershipOfInner;
+    private final boolean takeOwnershipOfInner;
+
+    // --------------------------------------------------------------------------------
+    //
+    // Constructors
+    //
+    // --------------------------------------------------------------------------------
 
     /**
      * create a local-sorting VCF writer, given an inner VCF writer to write to
@@ -62,16 +66,27 @@ public abstract class SortingVCFWriterBase implements VCFWriter {
      */
     public SortingVCFWriterBase(VCFWriter innerWriter, boolean takeOwnershipOfInner) {
         this.innerWriter = innerWriter;
-        this.queue = new PriorityQueue<VCFRecord>(50, new VariantContextComparator());
-        this.mostUpstreamWritableLoc = BEFORE_MOST_UPSTREAM_LOC;
         this.finishedChromosomes = new TreeSet<String>();
         this.takeOwnershipOfInner = takeOwnershipOfInner;
+
+        // has to be PriorityBlockingQueue to be thread-safe
+        // see http://getsatisfaction.com/gsa/topics/missing_loci_output_in_multi_thread_mode_when_implement_sortingvcfwriterbase?utm_content=topic_link&utm_medium=email&utm_source=new_topic
+        this.queue = new PriorityBlockingQueue<VCFRecord>(50, new VariantContextComparator());
+
+        this.mostUpstreamWritableLoc = BEFORE_MOST_UPSTREAM_LOC;
     }
 
     public SortingVCFWriterBase(VCFWriter innerWriter) {
         this(innerWriter, false); // by default, don't own inner
     }
 
+    // --------------------------------------------------------------------------------
+    //
+    // public interface functions
+    //
+    // --------------------------------------------------------------------------------
+
+    @Override
     public void writeHeader(VCFHeader header) {
         innerWriter.writeHeader(header);
     }
@@ -79,6 +94,7 @@ public abstract class SortingVCFWriterBase implements VCFWriter {
     /**
      * attempt to close the VCF file; we need to flush the queue first
      */
+    @Override
     public void close() {
         stopWaitingToSort();
 
@@ -86,27 +102,14 @@ public abstract class SortingVCFWriterBase implements VCFWriter {
             innerWriter.close();
     }
 
-    private void stopWaitingToSort() {
-        emitRecords(true);
-        mostUpstreamWritableLoc = BEFORE_MOST_UPSTREAM_LOC;
-    }
-
-    protected void emitSafeRecords() {
-        emitRecords(false);
-    }
-
-    protected void noteCurrentRecord(VariantContext vc) {
-        // did the user break the contract by giving a record too late?
-        if (mostUpstreamWritableLoc != null && vc.getStart() < mostUpstreamWritableLoc) // went too far back, since may have already written anything that is <= mostUpstreamWritableLoc
-            throw new IllegalArgumentException("Permitted to write any record upstream of position " + mostUpstreamWritableLoc + ", but a record at " + vc.getChr() + ":" + vc.getStart() + " was just added.");
-    }
 
     /**
      * add a record to the file
      *
      * @param vc      the Variant Context object
      */
-    public void add(VariantContext vc) {
+    @Override
+    public synchronized void add(VariantContext vc) {
         /* Note that the code below does not prevent the successive add()-ing of: (chr1, 10), (chr20, 200), (chr15, 100)
            since there is no implicit ordering of chromosomes:
          */
@@ -125,7 +128,43 @@ public abstract class SortingVCFWriterBase implements VCFWriter {
         emitSafeRecords();
     }
 
-    private void emitRecords(boolean emitUnsafe) {
+    /**
+     * Gets a string representation of this object.
+     * @return a string representation of this object
+     */
+    @Override
+    public String toString() {
+        return getClass().getName();
+    }
+
+    // --------------------------------------------------------------------------------
+    //
+    // protected interface functions for subclasses to use
+    //
+    // --------------------------------------------------------------------------------
+
+    private synchronized void stopWaitingToSort() {
+        emitRecords(true);
+        mostUpstreamWritableLoc = BEFORE_MOST_UPSTREAM_LOC;
+    }
+
+    protected synchronized void emitSafeRecords() {
+        emitRecords(false);
+    }
+
+    protected void noteCurrentRecord(VariantContext vc) {
+        // did the user break the contract by giving a record too late?
+        if (mostUpstreamWritableLoc != null && vc.getStart() < mostUpstreamWritableLoc) // went too far back, since may have already written anything that is <= mostUpstreamWritableLoc
+            throw new IllegalArgumentException("Permitted to write any record upstream of position " + mostUpstreamWritableLoc + ", but a record at " + vc.getChr() + ":" + vc.getStart() + " was just added.");
+    }
+
+    // --------------------------------------------------------------------------------
+    //
+    // private implementation functions
+    //
+    // --------------------------------------------------------------------------------
+
+    private synchronized void emitRecords(boolean emitUnsafe) {
         while (!queue.isEmpty()) {
             VCFRecord firstRec = queue.peek();
 
@@ -140,15 +179,6 @@ public abstract class SortingVCFWriterBase implements VCFWriter {
         }
     }
 
-    /**
-     * Gets a string representation of this object.
-     * @return a string representation of this object
-     */
-    @Override
-    public String toString() {
-        return getClass().getName();
-    }
-
     private static class VariantContextComparator implements Comparator<VCFRecord> {
         public int compare(VCFRecord r1, VCFRecord r2) {
             return r1.vc.getStart() - r2.vc.getStart();