ABSTRACT Aims This study investigates the relationship between frequency of attendance at Narcotics Anonymous and Alcoholics Anonymous (NA/AA) meetings and substance use outcomes after residential treatment of drug dependence. It was predicted that post-treatment NA/AA attendance would be related to improved substance use outcomes. Methods Using a longitudinal, prospective cohort design, interviews were conducted with drug-dependent clients (n = 142) at intake to residential treatment, and at 1 year, 2 years and 4–5 years follow-up. Data were collected by structured interviews. All follow-up interviews were carried out by independent professional interviewers. Findings Abstinence from opiates was increased throughout the 5-year follow-up period compared to pre-treatment levels. Clients who attended NA/AA after treatment were more likely to be abstinent from opiates at follow-up. Abstinence from stimulants increased at follow-up but (except at 1-year follow-up) no additional benefit was found for NA/AA attendance. There was no overall change in alcohol abstinence after treatment but clients who attended NA/AA were more likely to be abstinent from alcohol at all follow-up points. More frequent NA/AA attenders were more likely to be abstinent from opiates and alcohol when compared both to non-attenders and to infrequent (less than weekly) attenders. Conclusions NA/AA can support and supplement residential addiction treatment as an aftercare resource. In view of the generally poor alcohol use outcomes achieved by drug-dependent patients after treatment, the improved alcohol outcomes of NA/AA attenders suggests that the effectiveness of existing treatment services may be improved by initiatives that lead to increased involvement and engagement with such groups.