Serotonergic implication in panic disorder has been demonstrated by the efficacy of serotonin reuptake blockers in treatment. Fluoxetine, a potent 5-HT reuptake blocker, has been suggested to have anti-panic efficacy. This open study examines 30 patients (eight males and 22 females) with an average age of 36.9 years, ranging from 18 to 62, who were treated for eight weeks with fluoxetine (mean dose 20 mg per day). All patients fulfilled DSM-III-R criteria of panic disorder with agoraphobia as determined in a SCID interview schedule. Out of 28 patients who started medication, 64% of the patients completed the clinical trial and 36% of the patients dropped out of treatment because of increased anxiety or a lack of efficacy. Thirty-two percent of the patients had zero panic attacks by week 3. By the end of eight weeks of treatment, 48% of the patients had zero panic attacks. There was a significant reduction in anxiety and phobic avoidance and panic attacks. Tritiated platelet imipramine and paroxetine bindings revealed significantly lower maximal binding for patients with panic disorder in comparison with controls. Paroxetine Bmax showed a trend to increase in the direction of control values by the end of the trial.