Objective: To study the prevalence of psychopathology (Axis I and Axis II) in a population of fifty-five male adolescent sex offenders. Method: Measures included the NIMH Diagnostic Interview Schedule for Children (DISC-2), Diagnostic Interview for Borderlines (Revised) DIB-R, The Millon Adolescent Personality Inventory (MAPI) and the Structured Clinical Interview for DSM-III-R Personality Diagnoses (SCID II). Results: The average adolescent sex offender committed his first sexual offense at 10.5, (SD = 2.03) years of age, committed 5.4 sexual crimes and had 3.2 victims. The average sexual offender had 3.1 Axis I psychiatric diagnoses (DISC-2). The younger the age of onset of sexually offending behavior, the higher the number of Axis I psychiatric diagnoses. The most frequent diagnoses were conduct disorder (78.6 percent), anxiety disorders (40.5 percent), and mood disorders (31.0 percent). Seventy-five percent had two or more Axis I psychiatric diagnoses. The presence of borderline personality disorder (BPD) varied from 35-72 percent. The younger the age of onset of sexually offending behavior and the younger the offender was at the time of his own sexual victimization, the more likely he would endorse BPD symptoms on the DIB-R. On the SCIDII the average sex offender manifested five personality diagnoses with 68 percent having more than four diagnoses. Conclusion: Sexual offending behavior occurs in the context of severe psychiatric morbidity and character pathology requiring treatment strategies sensitive to the range of emotional and behavioral problems intrinsic to this population.