Adolescent male sex offenders who assaulted younger siblings (n = 32) were compared to those who offended against nonsibling children (n = 28). Data were based on responses to the Assessing Environments (III) Scale, Family-of-Origin Scale, Youth Self-Report, Buss-Durkee Hostility Inventory, Tennessee Self-Concept Scale, and the Beck Depression Inventory. Unlike many earlier studies of adolescent sex offenders, sexual offending/victimization histories were based on information collected from regular meetings rather than intake files or initial interviews; offenders' age, socioeconomic status (SES), and social desirability were examined to avoid potential confounds of these variables; victim age and gender were analyzed to ensure that comparisons between sibling and nonsibling offenders were not confounded by victim age or gender; and internal consistencies of the variables were verified with a larger clinical sample (n = 209). Adolescent sibling-incest offenders reported significantly more marital discord, parental rejection, physical discipline, negative family atmosphere, and general dissatisfaction with family relationships. Offenders against siblings were also more often victims of childhood sexual abuse and were more likely to have a younger child in their families. Results are discussed with respect to the etiology and treatment of adolescent sibling-incest offenders.