In a double-blind, randomized, parallel-group trial, we compared topiramate (TPM) with placebo as add-on therapy in patients with refractory partial epilepsy. TPM was titrated either to the target dosage of 800 mg/ day [400 mg twice daily (b.i.d)] or to the maximal tolerated dose if lower. Twenty-eight (28) patients were randomized to each treatment group. In the intent-to-treat analysis, the net median percent reduction relative to placebo in average monthly seizure rate was 54% for patients in the TPM group (p < 0.001). None of the placebo-treated patients and 43% of the patients treated with TPM experienced > or = 50% reduction in seizures (p = 0.001), and 36% of patients assigned to TPM had a 75-100% reduction in seizures (p < 0.01). Secondarily generalized seizures were also significantly reduced in the TPM group (p = 0.044). The most common adverse events (AE) reported in the TPM group were fatigue, impaired concentration, weight loss, dizziness, and paresthesias. AE occurring either during the rapid titration of TPM or at high dosages led 21% of TPM-treated patients to withdraw from the study. Half of these occurred during the titration study period. No serious AE or clinically important changes in clinical laboratory measures were observed. The present study further establishes the favorable profile and good benefit/risk ratio of TPM in resistant partial epilepsy.