Six affected members of two families with familial periodic cerebellar ataxia were relieved of their symptoms with oral acetazolamide. Phosphorus 31 (31P) nuclear magnetic resonance spectroscopy showed abnormal intracellular pH levels in the cerebellum of all subjects when they were not treated. These levels returned to normal with treatment. Cerebral pH values were also measured in one family and were normal before and after treatment. An additional 3 patients with similar attacks, but without a family history, had normal untreated pH values in the cerebellum and cerebrum.