Rates of return to active substance use after addiction treatment tend to be high; participation in 12-step fellowships (e.g., Alcoholics Anonymous) reduces relapse rates but many clients do not attend or attend for a short period only. This quasi-experimental study uses repeated measurement to explore the role of presence/absence of on-site 12-step meetings during treatment on post-treatment outcomes. Polysubstance-dependent clients (N = 219) recruited at a program with and one without 12-step on-site, were followed for one year post-treatment. On-site 12-step enhanced 12-step attendance, especially during treatment, and predicted continuous abstinence for the post-treatment year. Holding 12-step meetings on-site is a low-cost strategy that programs should consider to foster post-treatment remission maintenance. (Contains 3 tables.)