The aim of this study was to compare the characteristics of heroin or cocaine users who are not in contact with drug-treatment agencies in Switzerland to the characteristics of a group who are in treatment. A sample of 917 users of heroin and/or cocaine was recruited outside treatment settings by 31 Privileged Access Interviewers. Respondents were divided into a study group of 512 heroin and/or cocaine users not following any treatment, and a control group of 238 users who were following treatment. Respondents in the no-treatment group use drugs less frequently, are less likely to inject drugs, have a more social pattern of use and more often have the impression of controlling their drug use. They have less contact with the legal system and the police, are in a better social situation and more often perceive themselves to be in good health. In both groups, respondents whose main drug of use is heroin generally have a more problematic pattern of use than those who use mainly cocaine. There are no significant differences between the two groups regarding present HIV-risk behaviour and prevention. The data show no significant association between the duration of use of heroin or cocaine and signs for problem use. These findings support the hypothesis that drug users not in treatment and drug users in treatment are two distinct populations, in terms of profile of drug use and prevalence of social or health problems that are associated to it.