The notion that nonparent social support buffers the impact of parent depressive symptoms and substance use on adolescent depressive symptoms was tested in 142 Mexican American and 148 European American families with 12- through 15-year-old adolescents. Parent risk factors and adolescent nonparent adult social support were measured at baseline; adolescent symptoms were measured 1 year later. Analyses revealed significant interactions between social support, gender, ethnicity, and paternal depressive symptoms and substance use. Greater social support related to fewer depressive symptoms at follow-up. Mexican American girls benefited from more social support when fathers had lower depression and substance use scores; European American girls benefited from more social support when fathers had higher depression and substance use scores. No interaction effects emerged for boys or for mother depression and substance use. Variations based on gender, ethnicity, and parent risk factors suggest that nonparent social support might be different for girls and boys from diverse environments. (Contains 3 tables and 4 figures.)