The impact and course of additional diagnoses was examined in 126 patients undergoing cognitive-behavioral treatment for panic disorder. With the Anxiety Disorders Interview Schedule--Revised, a high comorbidity rate (51%) was observed at pretreatment. Pretreatment comorbidity was not predictive of premature termination, nor did it have a substantial impact on short-term treatment outcome. However, patients with comorbidity at posttreatment were more likely to have sought additional treatment over the follow-up interval. Although a significant and dramatic decline in the overall comorbidity rate was found at posttreatment (17%), at 24-month follow-up this rate had increased to a level (30.2%) that was no longer significantly different from pretreatment. This was despite the fact that patients maintained or improved on treatment gains for panic disorder over this interval. The implications of these findings for the treatment, conceptualization, and classification of emotional disorders are discussed.