Summary: A multicenter, double-blind, randomized, placebo-controlled study evaluated the efficacy and safety of gabapentin (Neurontin, GBP) as add-on therapy in 272 patients with refractory partial seizures who were receiving one to two standard antiepileptic drugs (AEDs). Efficacy assessments compared the frequency of partial seizures during the 12-week treatment phase (T) and the 12-week baseline period (B). The primary analysis compared data for patients receiving GBP 900 mg/day with placebo; the GBP 1,200-mglday group provided doseresponse data. Efficacy criteria were percentage of change in seizure frequency (PCH), responder rate (percentage of patients with ≥50% reduction in seizure frequency), and response ratio, where RRatio = (T B)/(T + B). Median PCH was–21.8% in the 900-mg/day group and −17.8% in the 1,200-mg/day group as compared with −0.3% in the placebo group. Responder rate was 22.9% in the 900-mg/day group and 10.1% in the placebo group (p = 0.020, Fisher's exact test). Adjusted mean Rratio was-0.136 in the 900-mg/day group and −0.025 in the placebo group (p = 0.0046, analysis of variance ANOVA). Results showed slightly greater improvement for the 1,20-mg/day than for the 900-mg/day group (RRatio =−0.157, responder rate 28.0%). Adverse events (AE) occurred in 69% of patients in the 900-mg/day group and in 64% in the 1,200-mg/day group as compared with 52% in patients receiving placebo as add-on therapy. The most frequent AE among patients treated with GBP were somnolence, dizziness, and fatigue. Clinical laboratory evaluations showed no clinically important trends and no evidence of hepatic or hematopoietic effects. GBP is safe and effective in treating some patients with refractory partial seizures.