We conducted a double-blind trial of isoniazid in 11 patients with essential tremor and four patients with other types of postural action tremor. The tremor had not been helped by beta-blockers or primidone. Isoniazid was given in doses up to 1, 200 mg daily, together with 100 mg pyridoxine, for four weeks. Results were assessed with subjective and objective scales. Only two patients with essential tremor appeared to benefit enough to continue the drug after the trial, and only one has benefited from its long-term use. Isoniazid may be useful in rare cases of essential tremor, but must be monitored carefully because of its toxicity.