1. The anticonvulsant potency of vigabatrin (gamma-vinyl GABA, GVG) was studied in an open trial in a group of 21 mentally handicapped patients with drug-resistant epilepsy. 2. With this treatment one third of these patients had more than 50% reduction in seizure frequency. The anticonvulsant effect appeared during the first month of therapy and was maintained during a 7-month study. The side effects were mild: mainly tiredness, aggressiveness, and ataxia. Other anticonvulsant drugs remained at baseline levels during GVG therapy. GVG was not found to modulate EEG recordings. 3. According to our results, GVG is effective for treating intractable epilepsy in mentally handicapped patients.