Felbamate (2-phenyl-1,3-propanediol dicarbamate) has a favorable preclinical profile in animal models of epilepsy. We present the results of a double-blind, randomized, placebo-controlled clinical trial in patients with partial seizures. Criteria for entry included a requirement for four or more partial seizures per month despite concomitant therapeutic blood levels of phenytoin and carbamazepine. Fifty-six patients (mean age, 31.4 years; 32 men, 24 women) completed the trial. The mean seizure frequencies for the 8-week periods analyzed were felbamate = 34.9, placebo = 40.2. Felbamate was statistically superior to placebo in seizure reduction, percent seizure reduction, and truncated percent seizure reduction. The mean felbamate dosage was 2,300 mg/d. Plasma felbamate concentrations ranged from 18.4 to 51.9 mg/l, mean = 32.5 mg/l. Adverse experiences during felbamate therapy were minor and consisted primarily of nausea and CNS effects. This trial indicates that felbamate is safe and effective in the treatment of comedicated patients with severely refractory epilepsy.