The present study evaluated the impact of a structured aftercare programme following residential treatment for severe alcohol and/or heroin dependent clients. Over 17 months, 77 participants were recruited to the study and allocated randomly to either a structured aftercare (SA) programme or to unstructured aftercare (UA) of crisis counselling on request. Independent clinicians interviewed participants and collaterals, at 4-month (median) intervals, for 12 months following residential treatment. SA compared to UA was associated with a fourfold increase in aftercare attendance and one-third the rate of uncontrolled principal substance use at follow-up. Participants who attended either type of aftercare relapsed a median of 134 days later than those who attended no aftercare. Overall, 23% of monitored participants remained abstinent throughout, 21% maintained controlled substance use and 56% relapsed, within a median of 36 days following residential treatment. The only significant predictor of days to relapse, controlling for age, was pretreatment use of additional substances. Participants with pretreatment additional substance use relapsed a median of 192 days earlier than those who had used no other substances. The degree of agreement between participant self-reports and collateral reports was fair-to-moderate and moderate among collaterals. Intention-to-treat analyses revealed significant and clinically meaningful reductions in substance use in this sample of severely dependent residential treatment clients. The generalizability of these results is limited because of significant differences in age and presenting substance between the study sample and other clients admitted to the service during the study. This latter group of younger, male, heroin-dependent clients with polydrug use who refuse opioid pharmacotherapy, are more likely to drop out of treatment or relapse early following treatment and continue to present a challenge to treatment services.