This study reports on rates and predictors of sex risk behavior among a sample of adolescent and young adult children of parents enrolled in methadone treatment for opiate addiction. Data are from 151 participants (80 males, 71 females) in the Focus on Families (FOF) project, a randomized trial of a family intervention and a study of the development of at-risk children. The study participants are children of parents enrolled in methadone treatment between 1990 and 1993. Participants were interviewed in 2005 when they ranged in age from 15 to 29 years. In the year prior to the follow-up, 79% of the males and 83% of females were sexually active, 26% of males and 10% of females had more than one partner in the prior year, and 34% of males and 24% of females reported having sex outside of a committed relationship. Twenty-four percent of males and 17% of females met criteria for high-risk sexual behavior, reporting casual or multiple partners in the prior year and inconsistent condom use. Participants in the intervention and control conditions did not differ significantly in terms of any measure of sex risk behavior examined. None of the measures of parent behavior and family processes derived from data at baseline of the FOF study predicted whether participants engaged in high-risk sex. Among measures derived from data collected at long-term follow-up, however, having ever met criteria for substance abuse or dependence predicted greater likelihood of high-risk sexual behavior, and being married or being in a romantic relationship was associated with lower likelihood of high-risk sexual behavior. The findings point to the important role of committed relationships in regulating sex risk behavior among this population, as well as heightened levels of sex risk behavior associated with substance abuse or dependence.