In 1996 the Illinois Criminal Justice Information Authority funded the Sex Offender Treatment Program at the Illinois Youth Center at Harrisburg (IYC-H) to provide intensive residential treatment and aftercare to juvenile sex offenders. This program has two components: the Sex Offender Treatment Unit (SOTU), a residential treatment unit located in IYC-H, and the Cook County Juvenile Parole District, which is responsible for the aftercare component. This evaluation focuses on SOTU. The overall SOTU treatment program has four phases. Pretreatment focuses on learning the rules for treatment group process; overcoming denial and accepting responsibility for sex offenses committed; and learning terminology and understanding concepts related to sex crimes, thinking errors, and risk factors. The primary goal of Phase 1 is for each youth to learn about the concept of a sexual assault cycle and to understand his individual sexual assault cycle. Treatment elements are similar to those in the pretreatment phase. During Phase 2, youth are expected to improve their understanding of the consequences of sexual offenses, learn the life factors leading to criminal behavior, and develop a plan to alter dysfunctional factors in their own lives. In Phase 3, the final phase, youth are expected to develop a specific plan for intervention in their personal offense cycle, exercise group leadership, and complete a relapse prevention plan. Evaluation recommendations pertinent to implementation focus on the treatment environment, program administration, and staffing issues. 11 tables, 3 figures, 116 references, and appended evaluation instruments