Clonazepam, a chlorinated derivative of nitrazepam, was administered to 10 children with absence seizures. Serum concentrations were measured after 8 weeks of treatment, at steady state. Seizure frequency reports and the 12-hour telemetered electroencephalogram were studied before and after 8 weeks of treatment to determine the frequency and duration of generalized spike-wave paroxysms. The clonazepam dosage ranged from 0.028 to 0.111 mg per kilogram and was reflected in serum levels ranging from 13 to 72 ng per milliliter, with an excellent correlation between dose and serum level. Eight of the 10 patients showed a significant decrease in seizure frequency, with three experiencing no seizures at all. Six patients had side effects, predominantly drowsiness and ataxia. This preliminary study shows clonazepam to be useful in the treatment of absence seizures in children and to merit further study.