Alcoholics Anonymous (AA) attendance is predictive of increased abstinence for many problem drinkers and treatment referral to AA is common. Strong encouragement to acquire an AA sponsor is likewise typical, and findings about the benefits associated with social support for abstinence in AA support this practice, at least indirectly. Despite this widespread practice, however, prospective tests of the unique contribution of having an AA sponsor are lacking. This prospective study investigated the contribution of acquiring an AA sponsor using a methodologically rigorous design that isolated the specific effects of AA sponsorship. Participants were recruited from AA and outpatient treatment. Intake and follow-up assessments included questionnaires, semi-structured interviews, and urine toxicology screens. Eligibility criteria limited prior treatment and AA histories to clarify the relationship of interest while, for generalizability purposes, broad substance abuse criteria were used. Of the 253 participants, 182 (72%) provided complete data on measures central to the aims of this study. Overall reductions in alcohol, marijuana, and cocaine use were found over 12-months and lagged analyses indicated that AA attendance significantly predicted increased abstinence. During early AA affiliation but not later logistic regressions showed that having an AA sponsor predicted increased alcohol-abstinence and abstinence from marijuana and cocaine after first controlling for a host of AA-related, treatment, and motivational measures that are associated with AA exposure or are generally prognostic of outcome.