The aim of this thesis is to describe the users visiting the syringe exchange clinic in Malmö with respect to what characterises the group, how they utilize the services of the clinic and how their patterns of participation relate to risk behaviour, physical and mental health and social development. The heterogeneity of the group has been captured by the use of Stimsons dimensions “integration in majority society” and “involvement in sub-culture”, yielding four different groups of syringe exchangers: “stables”, “loners”, “two-worlders” and “junkies”. The study rests on two sources of data: a register from the clinic including all syringe exchangers that have visited the clinic between 1989 and 2003, altogether 3660 individuals, and an interview of visitors at the clinic during 1995, including 496 persons. The results show that syringe exchangers, compared to other persons with severe addiction in Malmö, to a larger extent use amphetamine as their drug-of-choice, are older and inject more irregularly. The population visiting the clinic is heterogeneous with respect to integration in society and involvement in sub-cultures. The group classified as junkies do, as expected, display the highest inclination to share syringes and needles with other and hence have the most advanced risk behaviours. As to utilization of the programme, the results show that the longer the syringe exchangers stay in the programme, the more frequent they visit it. Also with respect to utilization-patterns, we find substantial variations within the studied group. Five categories are discernible: drop-in visitors with only one or two visits, sporadic visitors who in spite of contacts over a number of years never really establish a regular contact, intermittent visitors who have had contact over several years but display a very irregular visiting pattern, regular visitors who relatively fast establish a consistent contact with visits between uniform time intervals and frequent visitors who tend to stay longer than others and visit the clinic more often. Another aspect of utilization is to what extent the distribution of needles and syringes cover the needs of the visitors. With a strict definition of need, only a minority manages to cover their needs, but if we accept a more extensive individual re-use, around 90 percent of the average need is covered. Data also reveals that a very high proportion of the users on at least one occasion have visited the programme without syringe exchange taking place or any complementary service delivered. Basically these visits seem to be of a more social nature, reinforcing the contacts between the staff and the visitors. Data do not give any clear support for the basic assumption that syringe exchange reduce the incidence of HIV or hepatitis. Recent sharing of utensils or low coverage of syringe need through the programme do not predict a higher infection risk. Instead we find that the social contacts with the staff (without syringe exchange) function as a predictor of lower incidence. This indicates that the mechanisms may be more complex than just related to the provision of clean needles and syringes. The result shows that integration increase over time while sub-cultural involvement decreases. However, the patterns for different groups are very diverse and we find no evidence that more frequent contacts as such are related to increased integration. There is however, some evidence that social visits are positively related to increased integration. No support could be found for the assumption that the programme increases the number of severe addicts.