Twenty-five out of 41 (61%) patients with drug-resistant complex partial seizures showed an initial dramatic response to clobazam. Sixteen of these responders developed tolerance to the effects of clobazam so that only 9 (22%) maintained a dramatic response for 1 year. Factors which appeared important in predicting a dramatic response to treatment were a known aetiology for the epilepsy, the occurrence of complex partial seizures alone (i.e., without secondary generalisation) and the absence of mental retardation.