diff --git a/Lib/ensurepip/__init__.py b/Lib/ensurepip/__init__.py
index b6bc4b6..5ceda51 100644
--- a/Lib/ensurepip/__init__.py
+++ b/Lib/ensurepip/__init__.py
@@ -43,10 +43,18 @@
     """
     Bootstrap pip into the current Python installation (or the given root
     directory).
+
+    Note that calling this function will alter both sys.path and os.environ.
     """
     if altinstall and default_pip:
         raise ValueError("Cannot use altinstall and default_pip together")
 
+    # We deliberately ignore all pip environment variables
+    # See http://bugs.python.org/issue19734 for details
+    keys_to_remove = [k for k in os.environ if k.startswith("PIP_")]
+    for k in keys_to_remove:
+        del os.environ[k]
+
     # By default, installing pip and setuptools installs all of the
     # following scripts (X.Y == running Python version):
     #