In order to assess the potential role of intravenous drug users (IVDUs) in Amsterdam in the heterosexual transmission of HIV, we studied the sexual behaviour of 243 male and female IVDUs with regard to their private ('steady' and 'casual') and commercial partners. The majority of the study group appear heterosexually active, especially female IVDUs, who often have commercial as well as one or more private partners. Vaginal contact is the most frequent method of sexual contact; condoms are used infrequently with private partners but much more frequently with commercial partners. A history of sexually transmitted disease is common, not only among IVDU prostitutes but also among IVDUs without a history of prostitution. IVDUs who know that they are HIV-infected reported fewer commercial partners and more frequent condom use with private partners than IVDUs who are negative or do not know their test result. We conclude that IVDUs may form an important source for heterosexual transmission of HIV. Voluntary testing of IVDUs on a large scale, together with intensive counselling, may slow down the further spread of HIV from infected IVDUs to their private and commercial partners, but is unlikely to stop heterosexual transmission completely.