The purpose of this study was to conduct a longitudinal examination of predictors of sexual aggression among male juvenile offenders. Four hundred and four adolescent males between the ages of 14 and 17 years were recruited from juvenile probation offices to take part in a prospective study of substance use and sexual risk. At baseline, participants completed a series of questionnaires that assessed putative risk factors for sexual aggression. They then completed a measure of sexual aggression at the 6-month follow-up period. Correlational analyses revealed that participants who reported hard drug use, more frequent alcohol and marijuana use, and less severe offenses reported engaging in more severe sexual aggression. In addition, participants who reported higher impulsivity, sensation seeking, and externalizing behaviors also reported participating in more severe sexual aggression. When these variables were included in a regression analysis, only externalizing behaviors and severity of offense uniquely predicted severity of sexual aggression at the 6-month follow-up. (Contains 1 table.)