Four patients with cerebrotendinous xanthomatosis (CTX) underwent electrophysiologic investigations, which demonstrated impairment in the functioning of the peripheral nerves in all four cases. The changes consisted of slow motor and sensory conduction. The changes were most marked in the older subjects, in whom the disease was more advanced, and who also had clinical manifestations of mild peripheral neuropathy. We conclude that the peripheral nerves are damaged in CTX.