The results of a multicenter, double-blind, placebo-controlled clinical trial of the efficacy and safety of progabide (PGB) in the treatment of partial seizures are presented. This study was performed with a number of rigorous controls not usually present in clinical trials. These included uniform co-medication in which all patients received only phenytoin and carbamazepine; concentrations of these two drugs were maintained within narrow, predefined concentration ranges. There was no statistically significant difference between PGB and placebo in seizure frequency and seizure duration for most of the analyses performed. One patient was withdrawn from the study because of hepatotoxicity. PGB was associated with a significant inhibition of phenytoin but not carbamazepine clearance. The results of this study indicate that PGB was not a potent antiepileptic drug in this population of persons with intractable epilepsy.