Objective: To assess the impact of comorbid anxiety on treatment for adolescent depression in an effectiveness study of interpersonal psychotherapy for depressed adolescents (IPT-A). Method: A randomized clinical trial was conducted from April 1, 1999, through July 31, 2002. Sixty-three depressed adolescents, ages 12 to 18, received either IPT-A or treatment as usual delivered by school-based mental health clinicians. Adolescents with and without probable comorbid anxiety disorders were compared on depression and overall functioning. All analyses used an intent-to-treat design. Results: Comorbid anxiety was associated with higher depression scores at baseline (p less than 0.01) and poorer depression outcome posttreatment (p less than 0.05). IPT-A was nonsignificantly more effective in treating the depression of adolescents with comorbid anxiety (p = 0.07). Adolescents whose depression and functioning improved during the course of treatment also showed an improvement in anxiety (p less than 0.01), largely irrespective of treatment condition. Conclusions: Adolescents with comorbid depression and anxiety present with more severe depression and may be more difficult to treat. Structured treatments like IPT-A may be particularly helpful for comorbidly depressed adolescents as compared to supportive therapy. (Contains 2 tables and 2 figures.)