Heroin-assisted treatment has been examined broadly in Switzerland since 1994 within the context of scientific studies. The goal was to clarify the suitability of this treatment for opiate addicts whom previous therapy had failed to reach. Results of the initial research phase show that the target group could be reached for treatment extending 18 months with a satisfactory retention rate of 69%. The patients could improve their health and social situation during treatment and reduce illegal consumption of narcotics. Studies during the initial years primarily examined the viability of heroin-assisted treatment and its effects on the patients' psychosocial and somatic development. A second study phase ongoing since 1998 pursues the specific importance of medical and psychosocial treatment for patients' health and social development in heroin-assisted treatment. The focal point is the effort to optimise treatment of patients with comorbidity of psychiatric disorders and severe somatic diseases, particularly AIDS. Investigations carried out in Switzerland have been discussed broadly at an international level. Studies on heroin-assisted treatment are also being conducted at present in various countries. In future, co-operation should be further intensified with researchers on an international scale.