diff --git a/arena.go b/arena.go
index 1565fb1..201f075 100644
--- a/arena.go
+++ b/arena.go
@@ -19,13 +19,14 @@ package arenaskl
 
 import (
 	"errors"
+	"math"
 	"sync/atomic"
 	"unsafe"
 )
 
 // Arena should be lock-free.
 type Arena struct {
-	n   uint32
+	n   uint64
 	buf []byte
 }
 
@@ -55,24 +56,37 @@ func NewArena(size uint32) *Arena {
 }
 
 func (a *Arena) Size() uint32 {
-	return atomic.LoadUint32(&a.n)
+	s := atomic.LoadUint64(&a.n)
+	if s > math.MaxUint32 {
+		// Saturate at MaxUint32.
+		return math.MaxUint32
+	}
+	return uint32(s)
 }
 
 func (a *Arena) Reset() {
-	atomic.StoreUint32(&a.n, 1)
+	atomic.StoreUint64(&a.n, 1)
 }
 
 func (a *Arena) Alloc(size uint32, align Align) (uint32, error) {
+	// Verify that the arena isn't already full.
+	origSize := atomic.LoadUint64(&a.n)
+	if int(origSize) > len(a.buf) {
+		return 0, ErrArenaFull
+	}
+
 	// Pad the allocation with enough bytes to ensure the requested alignment.
 	padded := uint32(size) + uint32(align)
 
-	newSize := atomic.AddUint32(&a.n, padded)
+	// Use 64-bit arithmetic to protect against overflow.
+	newSize := atomic.AddUint64(&a.n, uint64(padded))
 	if int(newSize) > len(a.buf) {
+		// Doubles as a check against newSize > math.MaxUint32.
 		return 0, ErrArenaFull
 	}
 
 	// Return the aligned offset.
-	offset := (newSize - padded + uint32(align)) & ^uint32(align)
+	offset := (uint32(newSize) - padded + uint32(align)) & ^uint32(align)
 	return offset, nil
 }
 
diff --git a/arena_test.go b/arena_test.go
new file mode 100644
index 0000000..2881cd6
--- /dev/null
+++ b/arena_test.go
@@ -0,0 +1,48 @@
+/*
+ * Copyright 2017 Dgraph Labs, Inc. and Contributors
+ * Modifications copyright (C) 2017 Andy Kimball and Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package arenaskl
+
+import (
+	"math"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// TestArenaSizeOverflow tests that large allocations do not cause Arena's
+// internal size accounting to overflow and produce incorrect results.
+func TestArenaSizeOverflow(t *testing.T) {
+	a := NewArena(math.MaxUint32)
+
+	// Allocating under the limit throws no error.
+	offset, err := a.Alloc(math.MaxUint16, Align1)
+	require.Nil(t, err)
+	require.Equal(t, uint32(1), offset)
+	require.Equal(t, uint32(math.MaxUint16)+1, a.Size())
+
+	// Allocating over the limit could cause an accounting
+	// overflow if 32-bit arithmetic was used. It shouldn't.
+	_, err = a.Alloc(math.MaxUint32, Align1)
+	require.Equal(t, ErrArenaFull, err)
+	require.Equal(t, uint32(math.MaxUint32), a.Size())
+
+	// Continuing to allocate continues to throw an error.
+	_, err = a.Alloc(math.MaxUint16, Align1)
+	require.Equal(t, ErrArenaFull, err)
+	require.Equal(t, uint32(math.MaxUint32), a.Size())
+}