diff --git a/tests/test_batchtransform.py b/tests/test_batchtransform.py
index 17a9b7e3..b2a2c9c8 100644
--- a/tests/test_batchtransform.py
+++ b/tests/test_batchtransform.py
@@ -43,7 +43,7 @@ def return_price(data):
 
 
 class BatchTransformAlgorithmSetSid(TradingAlgorithm):
-    def initialize(self, sids):
+    def initialize(self, sids=None):
         self.history = []
 
         self.batch_transform = return_price(
@@ -111,15 +111,14 @@ class TestChangeOfSids(TestCase):
         )
 
     def test_all_sids_passed(self):
-        algo = BatchTransformAlgorithmSetSid(self.sids,
-                                             sim_params=self.sim_params)
+        algo = BatchTransformAlgorithmSetSid(sim_params=self.sim_params)
         source = DifferentSidSource()
         algo.run(source)
-        for df, date in zip(algo.history, source.trading_days):
+        for i, (df, date) in enumerate(zip(algo.history, source.trading_days)):
             self.assertEqual(df.index[-1], date, "Newest event doesn't \
                              match.")
 
-            for sid in self.sids:
+            for sid in self.sids[:i]:
                 self.assertIn(sid, df.columns)
 
             last_elem = len(df) - 1
diff --git a/zipline/utils/data.py b/zipline/utils/data.py
index 8eead051..b12fd367 100644
--- a/zipline/utils/data.py
+++ b/zipline/utils/data.py
@@ -32,29 +32,61 @@ class RollingPanel(object):
     Restrictions: major_axis can only be a DatetimeIndex for now
     """
 
-    def __init__(self, window, items, minor_axis, cap_multiple=2,
+    def __init__(self, window, items, sids, cap_multiple=2,
                  dtype=np.float64):
         self.pos = 0
         self.window = window
 
         self.items = _ensure_index(items)
-        self.minor_axis = _ensure_index(minor_axis)
+        self.minor_axis = _ensure_index(sids)
 
         self.cap_multiple = cap_multiple
         self.cap = cap_multiple * window
 
         self.dtype = dtype
         self.index_buf = np.empty(self.cap, dtype='M8[ns]')
-        self.buffer = pd.Panel(items=items, minor_axis=minor_axis,
-                               major_axis=range(self.cap),
-                               dtype=dtype)
+
+        self.buffer = self._create_buffer()
+
+    def _create_buffer(self):
+        return pd.Panel(items=self.items, minor_axis=self.minor_axis,
+                        major_axis=range(self.cap),
+                        dtype=self.dtype)
+
+    def _update_buffer(self, frame):
+        # Drop outdated, nan-filled minors (sids) and items (fields)
+        non_nan_cols = set(self.buffer.dropna(axis=1).minor_axis)
+        new_cols = set(frame.columns)
+        self.minor_axis = _ensure_index(new_cols.union(non_nan_cols))
+
+        non_nan_items = set(self.buffer.dropna(axis=1).items)
+        new_items = set(frame.index)
+        self.items = _ensure_index(new_items.union(non_nan_items))
+
+        new_buffer = self._create_buffer()
+        # Copy old values we want to keep
+        # .update() is pretty slow. Ideally we would be using
+        # new_buffer.loc[non_nan_items, :, non_nan_cols] =
+        # but this triggers a bug in Pandas 0.11. Update
+        # this when 0.12 is released.
+        # https://github.com/pydata/pandas/issues/3777
+        new_buffer.update(
+            self.buffer.loc[non_nan_items, :, non_nan_cols])
+
+        self.buffer = new_buffer
 
     def add_frame(self, tick, frame):
         """
         """
         if self.pos == self.cap:
             self._roll_data()
-        self.buffer.values[:, self.pos, :] = frame.ix[self.items].values
+
+        if set(frame.columns).difference(set(self.minor_axis)) or \
+           set(frame.index).difference(set(self.items)):
+            self._update_buffer(frame)
+
+        self.buffer.loc[:, self.pos, :] = frame.ix[self.items].T
+
         self.index_buf[self.pos] = tick
 
         self.pos += 1