We evaluated the effects of isoniazid on tremor in 13 patients with MS. Patients were evaluated before treatment, after 1 month of therapy (1,000 mg daily), and 1 month after the last dose. Ten patients improved on at least one of three methods of evaluation. Transient side effects were common, possibly because there was a high percentage of "slow acetylators"; dosage was reduced by one-half in five patients. Improvement was mild and did not result in significant functional improvement.