Substance use and sexual risk behaviors are common among adolescents. Although attention has focused primarily on alcohol use, less is known about the relationship between marijuana use and sexual risk behavior among high-risk youth. Since truant youth often experience problems in school, troubled family situations, and other psychosocial problems, they represent an important group of high-risk youth to study. Previous research suggests that truant youth are at considerable risk of continuing their troubled behavior in school and entering the juvenile justice system. It is also likely that truant youth are involved in marijuana use and sexual risk behavior at a higher rate, than the general youth population. Involving them in effective intervention services could reduce these risk behaviors. The current study presents 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 engaging in sexual risk behavior over time, and (2) the effects of a substance use BI on their marijuana use and sexual risk behavior. Analyses examined a growth model for parallel processes in marijuana use and sexual risk behavior, and an assessment of the effect of the intervention on linear and quadratic trends, and on subgroups of youth differing in their sexual risk behavior and marijuana use. Implications of the results for future research and service delivery are considered.