Seventy-five epilepsy patients with at least two complex partial seizures/month were treated with gamma-vinyl GABA (GVG) 3 g/day for 3 months. Forty-one patients (54%) showed a reduction of greater than or equal to 50% in seizures. The median monthly seizure frequency decreased from 11.5 to 4 seizures/month. Twenty percent of patients had an improvement in general performance without a significant reduction in seizures. The responders entered the second phase of the study, in which 28 patients were randomly allocated to 3 g/day and 25 patients to 1.5 g/day GVG under double-blind conditions. The dosage of 3 g/day appeared to be clearly more effective than 1.5 g/day. However, even with 1.5 g/day GVG the seizure frequency was significantly reduced as compared to baseline. Drowsiness was the most commonly observed side effect, and it diminished with continued treatment. In three cases side effects led to the withdrawal of GVG therapy.