Although topiramate, one of the newer drugs used in treating epilepsy, is effective in reducing seizure frequency and has a wide spectrum of action, it often induces intolerable adverse effects, predominantly related to the central nervous system. Information that would help document adverse reactions early, thus allowing topiramate doses to be adjusted during the drug titration and maintenance phases, could be obtained from electroencephalogram (EEG) studies. We studied the clinical effects and EEG changes induced by topiramate in patients with refractory partial epilepsy receiving the drug as add-on therapy. To exclude effects related to the other drugs and to epilepsy itself, we compared data from patients and healthy volunteers. After receiving topiramate, 22.6% of patients became seizure free and 29% had their seizures reduced by 50% or more. Topiramate nevertheless induced noteworthy adverse reactions, the main problems being sedative and cognitive changes. Also, in healthy volunteers, a single 100-mg dose of topiramate induced mild adverse reactions, mainly affecting concentration and attention, with difficulties in speech and writing. In patients with epilepsy, the EEG changes induced by topiramate consisted of increased delta and theta activities and decreased activity in the rapid bands. This recognizable topiramate-induced EEG pattern was again evident in the healthy volunteers, in whom we also detected a significant reduction in the alpha frequency rhythm. Our results confirm that topiramate needs to be introduced gradually while patients undergo close neuropsychologic and neurophysiologic monitoring to detect adverse sedative and cognitive reactions early. The EEG correlate of these events seems to be increased activity in the slower frequency bands.