We studied an autoantibody (called anti-Tr), found in the serum and CSF of five patients with paraneoplastic cerebellar degeneration (PCD) and Hodgkin's disease (HD). Anti-Tr antibodies labelled the cytoplasm of Purkinje cells of human and rat cerebellum. The molecular layer of rat cerebellum showed a characteristic dotted pattern suggestive of immunoreactivity of dendritic spines of Purkinje cells. Patients with cerebellar disorders without HD (159) or HD without PCD (30) did not harbor anti-Tr antibodies. Immunoblots of human Purkinje cells or rat and mouse cerebellum were negative. Anti-Tr antibodies, as defined in this study, appear specific for HD-associated PCD. The immunohistochemical pattern described in the rat cerebellum coupled with the absence of reactivity in the immunoblot may be used to identify anti-Tr antibodies.