diff --git a/public/java/src/org/broadinstitute/sting/gatk/walkers/annotator/AverageAltAlleleLength.java b/public/java/src/org/broadinstitute/sting/gatk/walkers/annotator/AverageAltAlleleLength.java
new file mode 100644
index 000000000..d6768cb37
--- /dev/null
+++ b/public/java/src/org/broadinstitute/sting/gatk/walkers/annotator/AverageAltAlleleLength.java
@@ -0,0 +1,92 @@
+package org.broadinstitute.sting.gatk.walkers.annotator;
+
+import org.broadinstitute.sting.gatk.contexts.AlignmentContext;
+import org.broadinstitute.sting.gatk.contexts.ReferenceContext;
+import org.broadinstitute.sting.gatk.refdata.RefMetaDataTracker;
+import org.broadinstitute.sting.gatk.walkers.annotator.interfaces.ActiveRegionBasedAnnotation;
+import org.broadinstitute.sting.gatk.walkers.annotator.interfaces.AnnotatorCompatible;
+import org.broadinstitute.sting.gatk.walkers.annotator.interfaces.ExperimentalAnnotation;
+import org.broadinstitute.sting.gatk.walkers.annotator.interfaces.InfoFieldAnnotation;
+import org.broadinstitute.sting.utils.genotyper.PerReadAlleleLikelihoodMap;
+import org.broadinstitute.variant.variantcontext.Allele;
+import org.broadinstitute.variant.variantcontext.Genotype;
+import org.broadinstitute.variant.variantcontext.GenotypesContext;
+import org.broadinstitute.variant.variantcontext.VariantContext;
+import org.broadinstitute.variant.vcf.VCFHeaderLineType;
+import org.broadinstitute.variant.vcf.VCFInfoHeaderLine;
+
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * Created by IntelliJ IDEA.
+ * User: chartl
+ * Date: 1/3/13
+ * Time: 11:36 AM
+ * To change this template use File | Settings | File Templates.
+ */
+public class AverageAltAlleleLength extends InfoFieldAnnotation implements ActiveRegionBasedAnnotation, ExperimentalAnnotation {
+
+    public List<VCFInfoHeaderLine> getDescriptions() {
+        return Arrays.asList(new VCFInfoHeaderLine(getKeyNames().get(0), 1, VCFHeaderLineType.Float, "Average Allele Length"));
+    }
+
+    public List<String> getKeyNames() { return Arrays.asList("AAL"); }
+
+    public Map<String, Object> annotate(final RefMetaDataTracker tracker,
+                                        final AnnotatorCompatible walker,
+                                        final ReferenceContext ref,
+                                        final Map<String, AlignmentContext> stratifiedContexts,
+                                        final VariantContext vc,
+                                        final Map<String, PerReadAlleleLikelihoodMap> perReadAlleleLikelihoodMap ) {
+        if ( !vc.hasLog10PError() )
+            return null;
+
+        final GenotypesContext genotypes = vc.getGenotypes();
+        if ( genotypes == null || genotypes.size() == 0 )
+            return null;
+
+        Map<String, Object> map = new HashMap<String, Object>();
+
+        double length = getMeanAltAlleleLength(vc);
+        map.put(getKeyNames().get(0),String.format("%.2f",length));
+        return map;
+    }
+
+    public static double getMeanAltAlleleLength(VariantContext vc) {
+        double averageLength = 1.0;
+        if ( ! vc.isSNP() && ! vc.isSymbolic() ) {
+            // adjust for the event length
+            int averageLengthNum = 0;
+            int averageLengthDenom = 0;
+            int refLength = vc.getReference().length();
+            for ( Allele a : vc.getAlternateAlleles() ) {
+                int numAllele = vc.getCalledChrCount(a);
+                int alleleSize;
+                if ( a.length() == refLength ) {
+                    // SNP or MNP
+                    byte[] a_bases = a.getBases();
+                    byte[] ref_bases = vc.getReference().getBases();
+                    int n_mismatch = 0;
+                    for ( int idx = 0; idx < a_bases.length; idx++ ) {
+                        if ( a_bases[idx] != ref_bases[idx] )
+                            n_mismatch++;
+                    }
+                    alleleSize = n_mismatch;
+                }
+                else if ( a.isSymbolic() ) {
+                    alleleSize = 1;
+                } else {
+                    alleleSize = Math.abs(refLength-a.length());
+                }
+                averageLengthNum += alleleSize*numAllele;
+                averageLengthDenom += numAllele;
+            }
+            averageLength = ( (double) averageLengthNum )/averageLengthDenom;
+        }
+
+        return averageLength;
+    }
+}