BACKGROUND Many investigators have reported that panic disorder (PD) patients with comorbid major depression (MD) have more severe symptoms and a poorer response to treatment than patients with PD alone. It is not known if this is due to a distinct and more serious underlying disorder in these patients or simply a result of the simultaneous presence of the two disorders. METHODS Nondepressed patients presenting for treatment of panic disorder with agoraphobia (PDA) were studied before treatment (N = 180) and after 4 weeks of treatment with adinazolam sustained release (N = 89) or placebo (N = 91). Twenty-nine percent (N = 53) of the patients had a past history of MD. Symptom severity and treatment outcome were compared in patients with primary, secondary, single, recurrent, or no past MD. RESULTS There were no consistent differences in symptom severity or treatment outcome in patients with a past history of primary, secondary, or single episode MD compared with patients with no history of MD. However, a small number of patients with history of recurrent MD exhibited consistently greater symptom severity and poorer response to treatment than patients with no history of MD. CONCLUSIONS The greater severity and worse outcome of comorbid PD and MD observed in earlier studies are more likely due to the simultaneous presence of the two disorders than to a more serious and enduring underlying disorder. However, our results suggest that recurrent MD may indicate a more serious condition in patients with PDA. This possibility warrants further study.