Whereas the treatment outcome research literature for adolescent alcohol and other drug abuse has shown recent advances (R. J. Williams, S. Y. Chang, & Addiction Centre Adolescent Research Group, 2000), significant knowledge gaps remain. A. E. Kazdin (2001) recently observed that one of the key questions for the field is to identify if client characteristics meaningfully mediate or moderate treatment outcome. There is support from the adolescent clinical literature that internalizing and externalizing personality subtypes are related to the onset and course of youth substance use disorders (D. B. Clark & O. G. Bukstein, 1998). The study extends this literature by examining the association of drug use behaviors outcome and subtyped adolescents (internalizers and externalizers; n = 141) who sought treatment at a 12-Step program. The analysis also includes a community-based control group (n = 94). Specifically, we examined the association of subtype and treatment retention and short-term (Year 1) and long-term (Year 4 and Year 5.5) drug involvement outcomes. Externalizers consistently showed poorer outcomes, including poorer treatment retention and greater drug use and drug disorder symptoms at each follow-up point. The treatment implications of the study are discussed.