The long-term efficacy of primidone (375-750 mg/day) in essential tremor was evaluated prospectively in 11 patients who had shown a favorable response to 4-week treatment with the drug under placebo-controlled conditions. On accelerometric evaluation, the magnitude of tremor after 3, 6, and 12 months on primidone was still significantly reduced compared with the initial placebo period. After discontinuation of primidone, tremor amplitude reverted to the placebo levels. Some loss of efficacy during long-term administration, however, was suggested by the results of self-assessment, physician's assessment, and performance tests. Three patients discontinued prematurely the drug because the sedative effects outweighed the potential therapeutic benefit. Side effects (especially drowsiness and sedation) were common at 4 weeks and 3 months but tended to subside thereafter. It is concluded that primidone retains at least part of its tremorolytic effect for up to 1 year, although the overall clinical benefit is limited in most patients.