This study investigated differences in prevalence and determinants of HIV infection, and in recent risk behaviour (previous 6 months) among injecting drug users (IDUs) who are in contact with different types of services for IDUs in Berlin. Participants (n = 557) were recruited from drug-free long-term treatment centres, a storefront agency and a syringe exchange bus. HIV seroprevalence was lowest (3.9%) at the treatment centres, and highest among IDUs at the storefront agency (20.7%). In logistic regression, independent risk factors for HIV infection were duration of injecting drug use, borrowing syringes in prison, sex with HIV-positive partners, and prostitution. Syringe sharing in prison was the most important independent determinant of HIV infection among all three subpopulations of IDUs. Participants entering long-term treatment were most likely, and IDUs at the syringe exchange bus were least likely to have borrowed and passed on syringes in the previous 6 months. In logistic regression, site of recruitment was independently associated with recent borrowing of syringes, but not with condom use. Injection of drugs other than heroin only, and injecting in prisons, were also independent predictors of recent borrowing. The results indicate that IDUs entering treatment form an important target group for health education. There is a need for AIDS prevention measures in prisons. The comparatively low levels of recent injection risk behaviour among IDUs at the syringe exchange bus suggest that this type of intervention may be effective in harm reduction.