In a group of patients studied for 12 months after myocardial infarction plasma levels of phenytoin were measured whilst the patients received phenytoin 300–400 mg per day or placebo therapy. Two sub-groups of patients treated with phenytoin were identified, based on the steady state level of plasma phentyoin concentration. Patients who had achieved phenytoin plasma concentrations of 10 µg/ml or above, had a significantly lower mortality than patients who had plasma concentration below this level or than patients treated with placebo. Ventricular premature beats occurred significantly less often in the phenytoin treated group than among the comparative group of patients. The significance of these findings is discussed.