Event history analysis was used to test a developmental model of the timing of first sexual intercourse in the Oregon Yough Study sample of adolescent males at risk for delinquency. The event history models spanned grades 7-12 with yearly multimethod, multiagent measures. A 3-step mediational model of predictors was tested, including contextual and process factors and boys’ characteristics. Pedictors included one-time measures of socioeconomic status, parental antisocial behavior, and time-varyingmeasures of parental transitoins, parental monitoring, deviant peer association, and the boys’ antisocial/delinquent beahvior, substance use, physical maturation, academic achievement, and anxiety. As predicted, antisocial/delinquent beahvior and substance use along with early physical maturity and parental transitions predictred early onset of sexual intercourse. Anxiety was related to delay of first intercourse.