A patient with high titers of the anti-Ri antibody died 3 years after a progressive course with ataxia, opsoclonus, dementia, and peripheral neuropathy. At autopsy, no tumor was found. The nervous system exhibited severe Purkinje cell loss and contained perivascular and interstitial inflammatory infiltrates, particularly involving the brainstem. B and CD4 cells predominated in the perivascular spaces and CD8 cells in the interstitial infiltrates. Complement reactivity and natural killer cells were present and predominated in areas with more intense inflammatory infiltrates. Deposits of IgG were detected in the cytoplasm and nuclei of some neurons, particularly those in the brainstem tegmentum. The proportion of anti-Ri IgG in the total IgG extracted from various areas of the brain, serum, and cerebrospinal fluid was determined by quantitative western blot analysis. Anti-Ri reactivity was identified in immunoblots of all regions of the brain, but it predominated in basis pontis and dorsal mesencephalon. Our findings support the hypothesis of an autoimmune basis for the disorder and suggest that an antibody-mediated mechanism may play a role in its pathogenesis.