Clonazepam was added to the treatment of patients with poorly controlled epilepsy in a double-blind trial and an open trial. Considerable improvement occurred with patients with myoclonic jerks and tonic-clonic convulsions, and with photosensitive epilepsy. Patients with atypical petit mal and focal epilepsies also improved. Drowsiness was initially common but lasted only a short time. No evidence was found for an action of clonazepam on the metabolism of other drugs, but treatment with phenobarbitone lowered serum concentrations of clonazepam. We conclude that clonazepam is particularly valuable in epilepsy with associated myoclonsu and in photosensitive epilepsy.