diff --git a/zipline/algorithm.py b/zipline/algorithm.py
index b2172c00..302366db 100644
--- a/zipline/algorithm.py
+++ b/zipline/algorithm.py
@@ -22,6 +22,7 @@ import pandas as pd
 from contextlib2 import ExitStack
 from pandas.tseries.tools import normalize_date
 import numpy as np
+import sys
 
 from itertools import chain, repeat
 from numbers import Integral
@@ -125,6 +126,7 @@ from zipline.utils.math_utils import (
     tolerant_equals,
     round_if_near_integer
 )
+from zipline.utils.pandas_utils import clear_dataframe_indexer_caches
 from zipline.utils.preprocess import preprocess
 from zipline.utils.security_list import SecurityList
 
@@ -2344,6 +2346,30 @@ class TradingAlgorithm(object):
         try:
             data = self._pipeline_cache.unwrap(today)
         except Expired:
+            # Try to deterministically garbage collect the previous result by
+            # removing any references to it. There are at least three sources
+            # of references:
+
+            # 1. self._pipeline_cache holds a reference.
+            # 2. The dataframe itself holds a reference via cached .iloc/.loc
+            #    accessors.
+            # 3. The traceback held in sys.exc_info includes stack frames in
+            #    which self._pipeline_cache is a local variable.
+
+            # We remove the above sources of references in reverse order:
+
+            # 3. Clear the traceback.
+            sys.exc_clear()
+
+            # 2. Clear the .loc/.iloc caches.
+            clear_dataframe_indexer_caches(
+                self._pipeline_cache._unsafe_get_value()
+            )
+
+            # 1. Clear the reference to self._pipeline_cache.
+            self._pipeline_cache = None
+
+            # Calculate the next block.
             data, valid_until = self._run_pipeline(
                 pipeline, today, next(chunks),
             )
diff --git a/zipline/utils/cache.py b/zipline/utils/cache.py
index 427a67d6..f86d4cef 100644
--- a/zipline/utils/cache.py
+++ b/zipline/utils/cache.py
@@ -1,7 +1,7 @@
 """
 Caching utilities for zipline
 """
-from collections import namedtuple, MutableMapping
+from collections import MutableMapping
 import errno
 import os
 import pickle
@@ -20,7 +20,7 @@ class Expired(Exception):
     """
 
 
-class CachedObject(namedtuple("_CachedObject", "value expires")):
+class CachedObject(object):
     """
     A simple struct for maintaining a cached object with an expiration date.
 
@@ -47,6 +47,9 @@ class CachedObject(namedtuple("_CachedObject", "value expires")):
         ...
     Expired: 2014-01-01 00:00:00+00:00
     """
+    def __init__(self, value, expires):
+        self._value = value
+        self._expires = expires
 
     def unwrap(self, dt):
         """
@@ -62,9 +65,13 @@ class CachedObject(namedtuple("_CachedObject", "value expires")):
         Expired
             Raised when `dt` is greater than self.expires.
         """
-        if dt > self.expires:
-            raise Expired(self.expires)
-        return self.value
+        if dt > self._expires:
+            raise Expired(self._expires)
+        return self._value
+
+    def _unsafe_get_value(self):
+        """You almost certainly shouldn't use this."""
+        return self._value
 
 
 class ExpiringCache(object):
diff --git a/zipline/utils/pandas_utils.py b/zipline/utils/pandas_utils.py
index 84017a41..1b81897a 100644
--- a/zipline/utils/pandas_utils.py
+++ b/zipline/utils/pandas_utils.py
@@ -166,3 +166,22 @@ def ignore_pandas_nan_categorical_warning():
             category=FutureWarning,
         )
         yield
+
+
+def clear_dataframe_indexer_caches(df):
+    """
+    Clear cached attributes from a pandas DataFrame.
+
+    By default pandas memoizes `iloc`, `loc` objects on DataFrames, resulting
+    in refcycles that can lead to unexpectedly long-lived DataFrames.  This
+    function attempts to clear those cycles.
+
+    Parameters
+    ----------
+    df : pd.DataFrame
+    """
+    for attr in ('_loc', '_iloc'):
+        try:
+            delattr(df, attr)
+        except AttributeError:
+            pass