In companion double-blind, placebo-controlled, dose-ranging trials performed in the United States, topiramate (TPM) daily target dosages of 200-1,000 mg/day were evaluated as add-on therapy in adults with refractory partial seizures with or without becoming secondarily generalized. Net reductions in median monthly seizure frequency (active drug minus placebo) with the most efficacious dosages of TPM were 35% in the low-dose trial and 40% in the high-dose trial. Substantial reductions in secondarily generalized seizures were also observed with TPM. TPM appears to be an efficacious new antiepileptic drug in the management of partial epilepsy.