A double-blind, placebo-controlled trial was undertaken to compare the effects of imipramine and clomipramine in the treatment of panic disorder with or without agoraphobia. The number of dropouts in the placebo-treated group was 7; in the imipramine-treated group, 4; and in the clomipramine treated group, 0. Ten subjects fulfilled the 12 weeks of treatment in the placebo group, 25 in the imipramine group, and 22 in the clomipramine group. To minimize dropouts because of side effects, a flexible dose regimen with a careful escalation of doses was applied. The maximal dose allowed was 250 mg/day. The mean (±SEM) daily doses reached were 124 ± 9 mg (range, 50–250 mg) of imipramine and 109 ± 8 mg (range, 25–200 mg) of clomipramine. At the end of the trial, the number of panic attacks as well as the anxiety between attacks (measured using the Hamilton Rating Scale for Anxiety) were markedly reduced in patients treated with either of the two antide-pressant drugs, but only slightly decreased in patients on placebo. With respect to all major outcome parameters, i.e., full panic attacks, total number of anxiety attacks (full plus mild), and anxiety between attacks, the effect of clomipramine was clearly and significantly superior to that of imipramine (p < 0.001, p < 0.002, and p < 0.002, respectively). Moderate intake of diazepam was allowed; in the clomipramine group (p < 0.006), but neither in the imipramine group nor in the placebo group, a significant decrement in diazepam intake was observed during the course of the trial. The finding that clomipramine may have a higher potency and/or efficacy than imipramine in the treatment of panic disorder supports the concept that the antipanic effect of antidepressant drugs is due to the influence of these compounds on serotonergic rather than noradrenergic neurotransmission.