Objective: Shame has long been seen as relevant to substance use disorders, but interventions have not been tested in randomized trials. This study examined a group-based intervention for shame based on the principles of acceptance and commitment therapy (ACT) in patients (N = 133; 61% female; M = 34 years old; 86% Caucasian) in a 28-day residential addictions treatment program. Method: Consecutive cohort pairs were assigned in a pairwise random fashion to receive treatment as usual (TAU) or the ACT intervention in place of 6 hr of treatment that would have occurred at that same time. The ACT intervention consisted of three 2-hr group sessions scheduled during a single week. Results: Intent-to-treat analyses demonstrated that the ACT intervention resulted in smaller immediate gains in shame, but larger reductions at 4-month follow-up. Those attending the ACT group also evidenced fewer days of substance use and higher treatment attendance at follow-up. Effects of the ACT intervention on treatment utilization at follow-up were statistically mediated by posttreatment levels of shame, in that those evidencing higher levels of shame at posttreatment were more likely to be attending treatment at follow-up. Intervention effects on substance use at follow-up were mediated by treatment utilization at follow-up, suggesting that the intervention may have had its effects, at least in part, through improving treatment attendance. Conclusions: These results demonstrate that an approach to shame based on mindfulness and acceptance appears to produce better treatment attendance and reduced substance use. (Contains 3 tables and 3 figures.)