Research has demonstrated that adolescent peer group affiliations are consistent predictors of substance use initiation and maintenance; it is less clear how adolescent "romantic" relationships influence substance use behavior. Data were drawn from the National Longitudinal Study of Adolescent Health. Participants in the final dataset for the current study included adolescents (321 males and 321 females) who were identified in reciprocated romantic relationships at Wave 1 (1994-1995; mean age 16.7 years) that were followed into young adulthood and reassessed at two different time points (Wave 2 in 1996, mean age 17.7, and Wave 3 in 2001-2002, mean age 23.1). Data were gathered from both partners, and included demographic variables, longitudinal measures of substance use (alcohol, tobacco, and marijuana), and relationship seriousness. Hierarchical linear modeling using SAS PROC MIXED were utilized to test for individual versus partner influences. Results revealed individual and partner effects for the prediction of alcohol and tobacco, although individual effects were generally greater than partner influences. For marijuana use, as self-reported relationship seriousness increased, future marijuana use decreased. These findings suggest the developmental significance of adolescent romantic relationships on the prediction of future substance use behavior during young adulthood. (Contains 2 tables and 2 figures.)