Objective: To examine the relationship between psychiatric disorders and sexual behaviors among adolescents receiving mental health treatment. Adolescents in mental health treatment have been found to have higher rates of HIV risk behavior than their peers, but data concerning the relationship between psychopathology and risk are inconsistent and limited. Method: Eight hundred and forty adolescents (56% female, 58% African American, mean age = 14.9 years) and their parents completed computerized assessments of psychiatric symptoms via the Computerized Diagnostic Interview Schedule for Children (Shaffer, 2000a, 2000b). Adolescents also reported on sexual risk behaviors (vaginal/anal sex, condom use at last sex) and completed urine screens for a sexually transmitted infection (STI). Results: Adolescents meeting criteria for mania, externalizing disorders (oppositional defiant, conduct, and attention-deficit/hyperactivity disorders), or comorbid for externalizing and internalizing disorders (major depressive, generalized anxiety, and posttraumatic stress disorders) were significantly more likely to report a lifetime history of vaginal or anal sex than those who did not meet criteria for any psychiatric disorder (odds ratio [OR] = 2.0, 2.3, and 1.9, respectively). Adolescents meeting criteria for mania were significantly more likely to have 2 or more partners in the past 90 days (OR = 3.2) and to test positive for a STI (OR = 4.3) relative to adolescents who did not meet criteria for a psychiatric disorder. Conclusions: The presence of internalizing and externalizing disorders, especially mania, suggests the need for careful screening and targeting of adolescent sexual behavior during psychiatric treatment. (Contains 1 figure and 3 tables.)