diff --git a/python/mergeVCFs.py b/python/mergeVCFs.py
index 93c30fed6..bca5db31c 100755
--- a/python/mergeVCFs.py
+++ b/python/mergeVCFs.py
@@ -12,6 +12,9 @@ def main():
     parser.add_option("-f", "--f", dest="fai",
                         type='string', default=None,
                         help="FAI file defining the sort order of the VCF")
+    parser.add_option("-a", "--assumeSorted", dest="assumeSorted",
+                        action='store_true', default=False,
+                        help="If provided, this assumes the input VCF files are themselves sorted, enabling a simple efficent merge")         
          
     (OPTIONS, args) = parser.parse_args()
     if len(args) == 0:
@@ -21,6 +24,44 @@ def main():
     if OPTIONS.fai <> None: order = faiReader.readFAIContigOrdering(OPTIONS.fai)
     #print 'Order', order
 
+    if OPTIONS.assumeSorted:
+        mergeSort(args, order)
+    else:
+        memSort(args, order)
+
+def cmpVCFRecords(order, r1, r2):
+    if order <> None:
+        c1 = order[str(r1.getChrom())]
+        c2 = order[str(r2.getChrom())]
+        orderCmp = cmp(c1, c2)
+        if orderCmp <> 0:
+            return orderCmp
+    return cmp(r1.getPos(), r2.getPos())
+
+def mergeSort(args, order):
+    #print 'MergeSort', args, order
+    header = None
+    
+    orderMap = []
+    for file in args:
+        #print file
+        openedFile = open(file)
+        for header, record, counter in lines2VCF(openedFile, extendedOutput = True, decodeAll = False):
+            orderMap.append([record, file])
+            break
+        openedFile.close()
+
+    #print orderMap
+    sortedOrderMap = sorted(orderMap, key=lambda x: x[0], cmp = lambda r1, r2: cmpVCFRecords(order, r1, r2))
+    #print sortedOrderMap
+
+    for headerLine in header: print headerLine
+    for file in map( lambda x: x[1], sortedOrderMap):
+        #print file
+        for record in lines2VCF(open(file), extendedOutput = False, decodeAll = False):
+            print record.format()
+        
+def memSort(args, order):
     header = None
     records = []
     for file in args:
@@ -28,16 +69,7 @@ def main():
         for header, record, counter in lines2VCF(open(file), extendedOutput = True, decodeAll = False):
             records.append(record)
 
-    def cmpVCFRecords(r1, r2):
-        if order <> None:
-            c1 = order[str(r1.getChrom())]
-            c2 = order[str(r2.getChrom())]
-            orderCmp = cmp(c1, c2)
-            if orderCmp <> 0:
-                return orderCmp
-        return cmp(r1.getPos(), r2.getPos())
-
-    records.sort(cmpVCFRecords) 
+    records.sort(lambda r1, r2: cmpVCFRecords(order, r1, r2)) 
     for line in formatVCF(header, records):
         #pass
         print line