Paraneoplastic cerebellar degeneration is a well-defined clinical entity typically associated with ovarian or breast carcinoma. Serum or CSF of these patients has been shown to contain autoantibodies against the cytoplasm of Purkinje cells. These antibodies have been referred to as anti-Yo. We report the case of a 67-year old woman with ovarian adenocarcinoma presenting with subacute cerebellar dysfunction. Her serum was found to react with the cytoplasm of Purkinje cells and HeLa cells. On Western blot analysis a 60 to 70 kD reaction was obtained. This suggests the presence of a classical anti-Yo antibody.