Prior research has found that the association between substance use and risky sexual behavior is stronger for female and relatively younger adolescents than their male and older counterparts. In this study, it was predicted that gender and age would moderate the association between self-reported marijuana use and unprotected sexual intercourse in a similar way. The study targeted adolescents that were being detained in a holding facility for juveniles in an urban county in the southeastern United States. The 272 detainees in the study had been apprehended for a status offense (truancy, running away) or delinquent offense (assault, theft). The results found that marijuana use was associated with unprotected sexual intercourse in a specific event among relatively young adolescent detainees, but not among their older counterparts. The hypothesis was not supported. The magnitude and nature of the association between marijuana use and unprotected sexual intercourse did not vary significantly between male and female detainees. Explanations for the age at event and marijuana use interaction include the stronger effects of marijuana on less experienced users, and the less normative use for the younger group. This study has yielded findings that have a clear implication for sexually transmitted disease (STD) prevention efforts with adolescent detainees. Findings indicate that these interventions should address marijuana use, especially when participants are relatively young adolescents. These findings and the substantial public health burden associated with STDs and substance use among adolescent detainees support the need for additional research on this topic. 2 tables, 19 references