We studied the effect of chenodeoxycholic acid in 17 patients with cerebrotendinous xanthomatosis. Before treatment, all subjects were symptomatic, with Achilles tendon xanthomas (in 15 of 17), cataracts (in 12 of 17), dementia (in 13 of 17), pyramidal-tract signs (in all 17), cerebellar dysfunction (in 13 of 17), mild peripheral neuropathy (in 7 of 17), electroencephalographic abnormalities (in 10 of 13), and abnormal cerebral computerized axial tomographic scans (in 10 of 12). After at least one year of chenodeoxycholic acid treatment (750 mg per day), dementia cleared in 10 subjects, and pyramidal and cerebellar signs disappeared in 5 and improved in another 8. Peripheral neuropathy was no longer detected in six. The electroencephalogram became normal in five and showed fewer abnormalities in another three subjects. Cerebral computerized axial tomographic scans improved in seven patients; the changes included the disappearance of a cerebellar xanthoma in one case. Concomitantly, mean plasma cholestanol levels declined threefold, and abnormal bile acid synthesis was suppressed. We conclude that long-term therapy with chenodeoxycholic acid may correct the biochemical abnormalities and arrest and possibly reverse the progression of cerebrotendinous xanthomatosis.