diff --git a/remotefilelog/repack.py b/remotefilelog/repack.py
--- a/remotefilelog/repack.py
+++ b/remotefilelog/repack.py
@@ -616,19 +616,23 @@
                 # TODO: Optimize the deltachain fetching. Since we're
                 # iterating over the different version of the file, we may
                 # be fetching the same deltachain over and over again.
-                # TODO: reuse existing deltas if it matches our deltabase
+                meta = None
                 if deltabase != nullid:
-                    deltabasetext = self.data.get(filename, deltabase)
-                    original = self.data.get(filename, node)
-                    size = len(original)
-                    delta = mdiff.textdiff(deltabasetext, original)
+                    deltaentry = self.data.getdelta(filename, node)
+                    delta, deltabasename, origdeltabase, meta = deltaentry
+                    size = meta.get(constants.METAKEYSIZE)
+                    if (deltabasename != filename or origdeltabase != deltabase
+                        or size is None):
+                        deltabasetext = self.data.get(filename, deltabase)
+                        original = self.data.get(filename, node)
+                        size = len(original)
+                        delta = mdiff.textdiff(deltabasetext, original)
                 else:
                     delta = self.data.get(filename, node)
                     size = len(delta)
+                    meta = self.data.getmeta(filename, node)
 
                 # TODO: don't use the delta if it's larger than the fulltext
-                # TODO: don't use the delta if the chain is already long
-                meta = self.data.getmeta(filename, node)
                 if constants.METAKEYSIZE not in meta:
                     meta[constants.METAKEYSIZE] = size
                 target.add(filename, node, deltabase, delta, meta)
diff --git a/tests/test-remotefilelog-repack.t b/tests/test-remotefilelog-repack.t
--- a/tests/test-remotefilelog-repack.t
+++ b/tests/test-remotefilelog-repack.t
@@ -454,3 +454,30 @@
   $ hg repack --incremental --debug --config packs.maxpacksize=512
   removing oversize packfile $TESTTMP/hgcache/master/packs/a2731c9a16403457b67337a620931797fce8c821.datapack (365 bytes)
   removing oversize packfile $TESTTMP/hgcache/master/packs/a2731c9a16403457b67337a620931797fce8c821.dataidx (1.21 KB)
+
+Do a repack where the new pack reuses a delta from the old pack
+  $ clearcache
+  $ hg prefetch -r '2::3'
+  2 files fetched over 1 fetches - (0 misses, 100.00% hit ratio) over * (glob)
+  $ hg repack
+  $ hg debugdatapack $CACHEDIR/master/packs/*.datapack
+  $TESTTMP/hgcache/master/packs/abf210f6c3aa4dd0ecc7033633ad73591be16c95:
+  x:
+  Node          Delta Base    Delta Length  Blob Size
+  1bb2e6237e03  000000000000  8             8
+  d4a3ed9310e5  1bb2e6237e03  12            6
+  
+  Total:                      20            14        (42.9% bigger)
+  $ hg prefetch -r '0::1'
+  2 files fetched over 1 fetches - (0 misses, 100.00% hit ratio) over * (glob)
+  $ hg repack
+  $ hg debugdatapack $CACHEDIR/master/packs/*.datapack
+  $TESTTMP/hgcache/master/packs/09b8bf49256b3fc2175977ba97d6402e91a9a604:
+  x:
+  Node          Delta Base    Delta Length  Blob Size
+  1bb2e6237e03  000000000000  8             8
+  d4a3ed9310e5  1bb2e6237e03  12            6
+  aee31534993a  d4a3ed9310e5  12            4
+  1406e7411862  aee31534993a  12            2
+  
+  Total:                      44            20        (120.0% bigger)