The safety of add-on lamotrigine (LTG, to 500 mg/day) was evaluated in 446 patients with partial seizures (334 LTG, 112 placebo) in a 6-month, multicenter, randomized, double-blind, parallel-group study. LTG had no clinically important effect on vital signs, electrocardiogram, body weight, clinical laboratory results, or physical and neurologic examination results. Dizziness, diplopia, ataxia, blurred vision, and somnolence occurred significantly more often with LTG treatment than with placebo. Most symptoms were mild or moderate and resolved with time. Eight percent of patients in both the LTG and placebo groups withdrew as a result of adverse events (AE). Three rashes in patients receiving LTG were judged serious by the investigators, and one, diagnosed as Stevens-Johnson syndrome, resulted in hospitalization; all resolved with discontinuation of LTG. Significantly more patients treated with LTG than with placebo improved as measured by the Investigators' Global Evaluations. LTG was well tolerated when administered as add-on treatment of epilepsy.