Since January 1994, heroin-assisted treatment for opiate addicts has been available in Switzerland. This is the first report of the long-term effects of this form of treatment. The report examines subjects who entered a study involving medical prescription of opiates (Projekt zur ärztlichen Verschreibung von Betäubungsmitteln; PROVE) in Switzerland between January 1994 and March 1995 (n = 366). Opiates were dispensed in eight treatment centres. A follow-up was conducted 6 years after treatment entry. Two groups were assessed: clients who have continuously been on heroin-assisted treatment since entry into the PROVE study or who re-entered this treatment, and ex-clients who had discontinued heroin-assisted treatment at the time of follow-up. Two kinds of comparisons were conducted. Firstly, conditions at treatment entry were compared to 6-year follow-up outcomes, and secondly, outcomes were compared between clients still on heroin-assisted treatment and those who had been discharged. It was found that 46% of the clients still alive were on heroin-assisted treatment at the time of follow-up. A comparison of the present living conditions showed very little difference between those in treatment and those who had terminated treatment. Compared to the situation at entry, the results of the follow-up showed a significant decrease in the use of illegal substances, illegal income and most other variables concerning social conditions, but they also showed an increase in unemployment and reliance on social benefits. Heroin-assisted treatment is thus efficacious in the long-term course of treatment and is still effective after termination of treatment with respect to living conditions and use of illicit substances.