The purpose of this study was to look at the efficacy and side effect profile of topiramate in a neurology unit. Using case notes, 94 patients who had been treated with topiramate were identified: 48 patients had taken part in clinical trials of topiramate, 46 received topiramate once licensed. Of these patients 24% had a greater than 50% decrease in seizure frequency. Patients with primary generalized epilepsy (n = 12) had a greater reduction in seizures compared with those with partial epilepsies (n = 70) P > 0.03. There was a high incidence (41%) of side effects, particularly psychiatric problems, leading to withdrawal of therpay in 41% of patients. Seven patients were admitted to hospital as a result of psychotic symptoms or depression. The incidence of psychotic symptoms (12%) was significantly higher for patients receiving topiramate compared with 191 patients attending the department on gabapentin (0.5%) and 270 patients attending the department on lamotrigine (0.7%) P < 0.001. 'Abnormal thinking', consisting of mental slowing and word-finding difficulties, occurred in 31%. The incidence could be significantly reduced by using 25 mg dose increments fortnightly as opposed to 100 mg weekly (P > 0.03). Although topiramate is an effective antiepileptic drug, its use is accompanied by a high incidence of particularly psychiatric side effects.