Summary:Seven of 63 children (11%) treated with clobazam (CLB) for refractory epilepsy developed a severe behavior disorder. This disorder was characterized by aggressive agitation, self injurious behavior, insomnia, and incessant motor activity occurring between 10 and 55 days after initiation of drug therapy. The affected children were relatively young (mean age 6.4 years) and developmentally disabled (four were autistic and two had isolated mental retardation). The disorder occurred with a short latency after initiation of therapy and at a relatively low dosage of CLB. Serum levels of other coadministered antiepileptic drugs were unchanged by the administration of CLB. One child was taking CLB monotherapy. This behavioral deterioration required the discontinuation of CLB, after which patients returned to their previous behavior within 3 weeks. After >3 years of follow-up all children continue to require multiple antiepileptic drugs but have not had a recurrence of this aggressive agitation. The mechanism of the behavioral change is unclear.