This randomized, double-blind, placebo-controlled UK trial evaluated the effect of topiramate as add-on therapy on seizure frequency, seizure severity, and quality of life in patients with epilepsy and intellectual disability. There were three phases: 4 weeks baseline, 18 weeks titration to 200-400 mg topiramate/day (adults) or 5-9 mg/kg/day (children), 12 weeks maintenance. Recruitment was low (88/120); analyses were underpowered. Seizure frequency varied enormously (median 17.7, maximum 1706.2). There was no significant difference in reduction in mean total seizure frequency or number of responders between the groups. Topiramate reduced seizure frequency by >30% from baseline (placebo 1%); post hoc analyses showed a trend toward significance (R ratio, P=0.052). There were no significant differences between the groups with respect to mean seizure severity or other outcome measures. Topiramate was generally well tolerated; body weight (P=0.015) and systolic blood pressure (P=0.043) were reduced. The study suggests that topiramate reduces seizure frequency in patients with epilepsy and intellectual disability without the added burden of behavior effects, and was potentially advantageous to physical well-being.