Self-reported histories of sexually transmissible diseases (STDs) and HIV serostatus were investigated as part of a study of HIV risk behaviour in a sample of 1245 Syndey injecting drug users (IDUs) (mean age 27.5 years) both in and out of treatment. A high lifetime prevalence of STDs was reported in both men and women. For male IDUs, the lowest reported lifetime prevalence of STDs was in heterosexuals, with bisexuals intermediate and homosexuals reporting the highest prevalence. HIV seroprevalence followed the same pattern. For women, bisexuals had the highest reported STD history, heterosexual women were intermediate and homosexual women reported the lowest prevalence. Over one third of the bisexual women reported having been involved in prostitution. These data indicate that over one third of IDU men and over half of IDU women reported at least one STD in their lifetime. The high lifetime prevalence of STDs in IDUs indicates that this group is at increased risk of sexual transmission of HIV, given the importance of STDs as a cofactor. Reducing the prevalence of STDs in IDUs is a possible additional strategy to diminish the spread of HIV among IDUs and from them to non-IDU sexual contacts.