A conceptual model was tested in which mentoring relationships were hypothesized to reduce substance use both directly and indirectly through improvements in adolescents' self-perceptions and close relationships. The study included 928 young adolescents (M age = 12.25), all of whom applied to Big Brothers Big Sisters programs. The adolescents were randomly assigned to either the treatment or control group and administered questions at baseline and 18 months later. The hypothesized model was not substantiated particularly well with data from the entire sample but was strongly supported when it was estimated with a subgroup of youth who were in longer lasting relationships. Being matched for longer than 12 months had significant impacts on the frequency of substance use and on parental relationships. Consistent with our conceptual model, parental relationships mediated the relationship between mentoring and substance use. Implications of these findings for research and interventions are discussed. (Contains 4 tables, 5 figures and 5 footnotes.)