Chorea-acanthocytosis has been separated as a clinical entity different from Huntington's chorea, mainly based on the clinical findings, but the histopathological and biochemical features of chorea-acanthocytosis, especially of basal ganglia, have not been well established, because only two such autopsy cases have been reported. The case presented here was a 39-year-old man at autopsy, with 10 years duration of typical symptoms and signs of chorea-acanthocytosis. At autopsy, the abnormal histopathological findings in the central nervous system were mainly confined to the striatum, where the caudate nucleus and putamen showed severe and moderate atrophy, respectively. Morphometric evaluation of the numbers of small and large neurons in the striatum with the adjustment for the shrinkage produced in the disease processes was performed. The numbers of small neurons in the caudate nucleus and putamen were 1% and 20% of each control, respectively. On the other hand, the large neurons in the caudate nucleus showed a reduction of diameters without a decrease in number and those in the putamen showed a mild decrease in number. In the biochemical studies, marked decrease of substance P (SP) level without definite decrease of choline acetyltransferase and glutamic acid decarboxylase (GAD) activities in both caudate nucleus and putamen was found. Substantia nigra, where no evident histopathological abnormalities were found, showed definite decrease of GAD activity and SP level. In the peripheral nervous system, the lateral branch of deep peroneal nerve showed mild degree of axonal degeneration. Neurogenic muscular atrophy with severe and mild degrees was found in extensor digitorum brevis and quadriceps femoris muscles, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)