Five patients participated in a controlled discontinuation of phenytoin (PHT) and carbamazepine (CBZ) after a study in which all subjects had felbamate (FBM) added to both PHT and CBZ. Four subjects (three women and 1 man aged 23-36 years) completed the protocol. Mean total seizure frequency per day with PHT and CBZ was 1.33 +/- 0.93 (mean +/- SEM), decreasing to 0.87 +/- 0.71 with addition of FBM, and 0.82 +/- 0.78 after discontinuation of PHT. Only one subject tolerated discontinuation of CBZ; the other three had dosage reductions of 33, 54, and 63%. Toxicity attributable to FBM was not observed, and patients often described less severe seizures. Results from four refractory patients indicated that FBM was able to replace PHT and reduce the need for CBZ. In addition, as PHT dosages were reduced, FBM clearance decreased 21%. As the CBZ dosages were reduced. FBM clearance decreased an additional 16.5%.