Primidone given to a patient for epilepsy produced an unexpected reduction in benign familial tremor. Over the next eight years the drug was therefore tried in a prospective study of 20 other patients with benign familial tremor alone. Of these, six could not tolerate the drug because of vertigo and nausea but 12 obtained a good response, which in some cases was dramatic. Investigations in two patients suggested that the effect was mediated predominantly by derived phenylethylmalonamide, though primidone had some effect, since tremor recurred slightly on withdrawing the drug despite a constant or rising blood phenylethylmalonamide concentration. Primidone is highly effective in benign familial tremor. More patients with the condition are intolerant of the drug than are usually found with epilepsy.