Six patients with severe postural cerebellar tremor were studied in a double-blind, placebo-controlled, crossover trial with isoniazid. Each phase of the study lasted 4 weeks, and in the isoniazid phase patients received an increasing dose up to 1200 mg/d. Patients were studied with self-rating scales, quantitative tremor recording, and blinded ratings of videotapes. All patients improved by at least one method, three improved by all methods, and four improved sufficiently to want to continue with the drug after the trial was finished. There is a definite, but limited, therapeutic role for isoniazid.