Since cholecystokinin (CCK) is known to be anxiogenic in experimental animals and to induce panic attacks in humans, lymphocyte CCK-8 concentrations were measured in 15 patients with panic disorder and 15 age- and sex-matched healthy subjects. The patients' levels were measured again after a 30-day course of alprazolam therapy, 1.5 mg/day. The CCK-8 concentrations were significantly lower in the patients than in the control subjects and did not change after alprazolam therapy. There was no correlation between the peptide values and levels of anxiety or frequency and severity of panic attacks.