Attendance at 12-step self-help groups is frequently recommended as an adjunct to professional substance use disorder (SUD) treatment, yet patient dropout from these groups is common. This study assessed the prevalence, predictors, and treatment-related factors affecting dropout in the first year following treatment for 2,778 male patients. Of these, 91% (2,518) were identified as having attended 12-step groups either in the 90 days prior to, or during, treatment. At 1-year followup 40% had dropped out. A number of baseline factors predicted dropout. Importantly, patients who initiated 12-step behaviors during treatment were less likely to drop out. Further findings suggest patients at highest risk for dropout may be at lower risk if treated in a more supportive environment. Clinicians may decrease the likelihood of dropout directly, by screening for risk factors and focusing facilitation efforts accordingly, and indirectly, by increasing the supportiveness of the treatment environment, and facilitating 12-step involvement during treatment.