Injecting drug users (IDUs) play a disproportionate role in the spread of HIV given their injecting and sexual contacts, and thereby act as conduits between these risk groups. We investigated differences in risk behaviour and HIV seroprevalence in a Sydney sample of 1,245 IDUs. Significant differences were observed across sexual orientation in HIV serostatus for males, with homosexual men having the highest HIV seroprevalence rate (35%), bisexual men intermediate (12%) and heterosexual men lowest (3%). Sexual HIV risk behaviours were lowest for homosexual men, intermediate for bisexual men, and highest for heterosexual men in the case of condom use: however, for numbers of partners, seroprevalence, and anal sex the trends were reversed. There were no differences across sexual orientation for either sex for injecting drug risk behaviours. Both male and female respondents reported having more than 50% of sexual contacts while under the influence of drugs. This study suggests that risk reduction in the sexual domain has not generalized to the injecting risk domain regardless of sexual orientation, and demonstrates that sexual risk behaviours in IDUs are lowest in homosexual, intermediate in bisexual, and highest in heterosexual IDU men.