Gabapentin is an analogue of gamma aminobutyric acid (GABA) which has anticonvulsant properties in animals. In a multicentre, double-blind, placebo-controlled, parallel-group study of 1200 mg/day gabapentin as additional therapy in 127 patients with drug-resistant partial epilepsy, 25% of patients who received gabapentin had the number of partial seizures at least halved, compared with 9.8% of patients given placebo. The median reduction in partial seizure frequency during 12 weeks' treatment was 29.2% with gabapentin compared with 12.5% with placebo. The mean adjusted response ratio for gabapentin (-0.192) was significantly better than the ratio of -0.060 for placebo by analysis of variance. 62% of patients who received gabapentin reported mostly mild or moderate adverse effects compared with 41% on placebo; no interactions were observed between gabapentin and other standard anticonvulsants. Gabapentin is an effective additional treatment for patients with partial epilepsy refractory to standard therapy, is fairly well tolerated, and appears to have a favourable efficacy-to-toxicity ratio.