Clonazepam, a high-potency benzodiazepine marketed for the treatment of minor motor epilepsy, was used to treat 50 patients with panic disorder (N = 22) or agoraphobia with panic attacks (N = 28). Of the 50 patients, 41 had previously been poorly responsive to standard pharmacologic therapies. At a mean dose of only 1.9 (+/- 1.0) mg/day, 39 patients (78%) responded. No serious adverse effects were encountered. This study, although retrospective and uncontrolled, suggests that clonazepam, like alprazolam, may be effective in blocking panic attacks. A possible common mechanism for the two drugs as high-potency benzodiazepines is discussed.