diff --git a/src/abstractdataframe/join.jl b/src/abstractdataframe/join.jl
index 39b168cf9f..0ef7b7d4b6 100644
--- a/src/abstractdataframe/join.jl
+++ b/src/abstractdataframe/join.jl
@@ -94,8 +94,17 @@ function DataArrays.PooledDataVecs(df1::AbstractDataFrame,
         for i = 1:length(refs2)
             refs2[i] += (dv2.refs[i]) * ngroups
         end
-        # FIXME check for ngroups overflow, maybe recode refs to prevent it
-        ngroups *= (length(dv1.pool) + 1)
+
+        if typemax(eltype(refs1))/(length(refs1)^2) <= ngroups
+            # about to overflow, recode refs1 and refs2 to drop the
+            # unused column combinations and limit the pool size
+            dv1, dv2 = PooledDataVecs( refs1, refs2 )
+            refs1 = dv1.refs .+ 1
+            refs2 = dv2.refs .+ 1
+            ngroups = length(dv1.pool) + 1
+        else
+            ngroups *= (length(dv1.pool) + 1)
+        end
     end
     # recode refs1 and refs2 to drop the unused column combinations and
     # limit the pool size
diff --git a/src/groupeddataframe/grouping.jl b/src/groupeddataframe/grouping.jl
index e2d7dd340f..dfac904309 100644
--- a/src/groupeddataframe/grouping.jl
+++ b/src/groupeddataframe/grouping.jl
@@ -93,15 +93,33 @@ function groupby{T}(d::AbstractDataFrame, cols::Vector{T})
     x = copy(dv.refs) .+ dv_has_nas
     # also compute the number of groups, which is the product of the set lengths
     ngroups = length(dv.pool) + dv_has_nas
+    dense_pool = true
     # if there's more than 1 column, do roughly the same thing repeatedly
     for j = (ncols - 1):-1:1
+        dense_pool = false
         dv = PooledDataArray(d[cols[j]])
         dv_has_nas = (findfirst(dv.refs, 0) > 0 ? 1 : 0)
         for i = 1:nrow(d)
             x[i] += (dv.refs[i] + dv_has_nas- 1) * ngroups
         end
-        ngroups = ngroups * (length(dv.pool) + dv_has_nas)
-        # TODO if ngroups is really big, shrink it
+
+        if typemax(eltype(x))/(length(x)^2) <= ngroups
+            dense_pool = true
+            # about to overflow, recode x to drop the
+            # unused combinations and limit the pool size
+            dv = PooledDataArray( x )
+            dv_has_nas = (findfirst(dv.refs, 0) > 0 ? 1 : 0)
+            x = dv.refs .+ dv_has_nas
+            ngroups = length(dv.pool) + dv_has_nas
+        else
+            ngroups *= (length(dv.pool) + dv_has_nas)
+        end
+    end
+    if !dense_pool
+        dv = PooledDataArray( x )
+        dv_has_nas = (findfirst(dv.refs, 0) > 0 ? 1 : 0)
+        x = dv.refs .+ dv_has_nas
+        ngroups = length(dv.pool) + dv_has_nas
     end
     (idx, starts) = DataArrays.groupsort_indexer(x, ngroups)
     # Remove zero-length groupings
diff --git a/test/grouping.jl b/test/grouping.jl
index 184a178b4f..ec055f6506 100644
--- a/test/grouping.jl
+++ b/test/grouping.jl
@@ -29,4 +29,9 @@ module TestGrouping
     h(df) = g(f(df))
 
     @test combine(map(h, gd)) == combine(map(g, ga))
+
+    # groupby should handle the permutations caused by grouping over many columns
+    N = 10000
+    dfc1 = DataFrame(A=1:N, B=1:N, C=1:N, dfc1=ones(N))
+    groupby(dfc1, [:A,:B,:C])
 end
diff --git a/test/join.jl b/test/join.jl
index 60fbd38187..636d737090 100644
--- a/test/join.jl
+++ b/test/join.jl
@@ -66,4 +66,10 @@ module TestJoin
 
     # Cross joins don't take keys
     @test_throws ArgumentError join(df1, df2, on = :A, kind = :cross)
+
+    # Do a join that would overflow unless the pool was recoded
+    N = 10000
+    dfc1 = DataFrame(A = 1:N, B=1:N, C=1:N, dfc1=ones(N))
+    dfc2 = DataFrame(A = 1:N, B=1:N, C=1:N, dfc2=2*ones(N))
+    join(dfc1, dfc2, on=[:A,:B,:C])
 end