Substance use and sexual risk behaviors are common among adolescents, but research has focused attention on alcohol use. Much less is known about the relationship of marijuana use and sexual risk behavior among high-risk, especially truant, youths. We report interim findings from a NIDA-funded experimental, brief intervention (BI) study involving truant youths and their parents/guardians. Longitudinal data were analyzed to study (1) the relationships between the youths' marijuana use and sexual risk behavior and (2) the effects of a substance use BI on their marijuana use and sexual risk behavior. A growth model analysis for parallel processes was conducted to study relationships between marijuana use and sexual risk behavior, and to assess the overall BI effect on linear and quadratic trends in subgroups of youth differing in their sexual risk behavior and marijuana use. Implications of the results for future research and service delivery are considered.