Clinical and pathological studies are reported from investigation of a 27-year-old man with GM1 gangliosidosis who experienced a slowly progressive dystonia that began about age 4, primarily affected the face and limbs, and eventually became almost totally incapacitating. There was only mild intellectual deterioration; myoclonus, seizures, and macular cherry-red spots were never observed. Postmortem examination revealed intraneuronal storage, localized predominantly to the basal ganglia, in which neurons contained round, multilamellated inclusions. Golgi studies revealed meganeurites arising from medium spiny neurons. Other areas of the central nervous system appeared relatively unaffected, although small basilar dilatations were observed in scattered cortical pyramidal neurons and Purkinje cell dendrites showed focal swellings. Vacuolated cells of the reticuloendothelial system were observed, including Kupffer cells and histiocytes in the spleen, marrow, and intestinal tract. Biochemical analysis revealed a generalized beta-galactosidase deficiency with specific accumulation of GM1 ganglioside in the basal ganglia.