To examine whether perceived peer/parent norms or personal beliefs about adolescent substance use influence substance use among female adolescents with chronic medical conditions. Sixty-eight females reported on substance use, personal beliefs, and perceived peer/parent norms. Personal beliefs and perceived peer/parent norms were associated with adolescent’s current and future substance use. Although perceived peer norms accounted for variance in current substance use, only personal beliefs accounted for variance in future alcohol use. Targeting perceived peer norms may be effective for intervention efforts among adolescents endorsing current substance use, whereas alcohol use prevention efforts should target personal beliefs.