Forty-three patients completed a double-blind, placebo-controlled study of Gabapentin (GBP) as add-on therapy in partial and secondarily generalized seizures. All patients were followed for an initial 3-month baseline period, after which they were randomly allocated to receive either a placebo or 900 or 1,200 mg/day GBP for 3 months. A statistically significant difference in seizure frequency from the baseline to the treatment phase was noted between patients receiving placebo and GBP 1,200 mg, in whom seizure frequency decreased 57%. The GBP dosage of 900 mg appeared to be ineffective. A close relationship was observed between the serum GBP concentrations and the GBP dosage based on the seizure frequency. Serum GBP concentrations greater than 2 micrograms/ml resulted in a lower frequency of seizures. The adverse effects were minor and consisted mainly of transient drowsiness. GBP appears to be effective in the treatment of partial epileptic seizures in a dosage-related manner.