This study examined the prospective relations among family history density of alcoholism (FHD), adolescent family harmony, and young adults' alcohol and drug dependence. Family harmony was rated by mothers and fathers in adolescence, and young adults' substance dependence diagnoses were obtained through structured interviews. Higher FHD predicted lower adolescent family harmony, which in turn increased young adults' odds of being diagnosed with drug dependence (with and without alcohol dependence) compared to no diagnoses or to alcohol dependence only. Family harmony also interacted with FHD such that the protective effect of family harmony on young adults' drug dependence with or without alcohol dependence decreased as FHD rose, and was nonsignificant at high levels of FHD. The findings suggest the importance of distinguishing among alcohol and drug dependence disorders and examining their differential etiological pathways, and also suggest that the protective effects of harmonious family environments on substance dependence may be limited at high levels of FHD.