Mutual-aid support groups play a vital role in substance abuse treatment in the United States. A national survey of mutual-aid support groups for addiction was conducted to identify key differences between participants in recovery groups. Survey data indicate that active involvement in support groups significantly improves one's chances of remaining clean and sober, regardless of the group in which one participates. Respondents whose individual beliefs better matched those of their primary support groups showed greater levels of group participation, resulting in better outcomes as measured by increased number of days clean and sober. Religious respondents were more likely to participate in 12-step groups and Women for Sobriety. Nonreligious respondents were significantly less likely to participate in 12-step groups. Religiosity had little impact on SMART Recovery participation but actually decreased participation in Secular Organizations for Sobriety. These results have important implications for treatment planning and matching individuals to appropriate support groups.