Paraneoplastic cerebellar degeneration (PCD) is thought to be caused by an autoantibody against both tumor and neuronal tissue. Such autoantibodies are most frequently detected in patients with gynecological or breast cancer, and are designated as anti-Yo. We report here a patient with PCD whose underlying cancer could not be detected despite extensive tumor survey. IgG in her serum and cerebrospinal fluid reacted with the cytoplasm of cerebellar Purkinje cells immunohistochemically. On immunoelectron microscopy, the endoplasmic reticulum and Golgi complex were stained. Her IgG bound to the 58 kD band on immunoblots of cerebellar proteins. A reaction was also observed with the recombinant proteins deduced from the complementary DNA clone encoding a neuronal cell antigen reported by Sakai et al (Ann Neurol 28: 692, 1990). Based on these results, successful early resection of fallopian tube adenocarcinoma was performed. It is crucially important to characterize these PCD related autoantibodies for the early treatment of underlying malignant tumors.