OBJECTIVE We examined timing of first voluntary sexual intercourse as a joint function of parental separation during childhood and parental history of alcoholism. METHODS Data were drawn from a birth cohort of female like-sex twins (n = 569 African ancestry [AA]; n = 3,415 European or other ancestry [EA]). Cox proportional hazards regression was conducted predicting age at first sex from dummy variables coding for parental separation and parental alcoholism. Propensity score analysis was also employed to compare intact and separated families, stratified by predicted probability of separation. RESULTS Earlier sex was reported by EA twins from separated and alcoholic families, compared to EA twins from intact nonalcoholic families, with effects most pronounced through the age of 14 years. Among AA twins, effects of parental separation and parental alcoholism were largely nonsignificant. Results of propensity score analyses confirmed unique risks from parental separation in EA families, where consistent effects of parental separation were observed across predicted probability of separation. For AA families, there was poor matching on risk factors presumed to predate separation, which limited interpretability of survival-analytic findings. CONCLUSIONS In European American families, parental separation during childhood is an important predictor of early-onset sex, beyond parental alcoholism and other correlated risk factors. To characterize risk for African Americans associated with parental separation, additional research is needed where matching on confounders can be achieved.