HIV prevention and risk reduction are especially salient and timely issues for women, particularly among those who are drug-involved or who exchange sex for drugs or money. Studies suggest that HIV-prevention measures can be effective with highly vulnerable women, and have the potential to produce significant reductions in risk behaviours among both HIV-negative and HIV-positive women. Within this context, this paper examines risk behaviours and HIV serostatus among 407 drug-involved women sex workers in Miami, Florida, and investigates the effects of participation in HIV testing, counselling, and a risk-reduction intervention on subsequent behavioural change among this population. Overall, at follow-up, the HIV-positive women were 2.4 times more likely than the HIV-negative women to have entered residential treatment for drug abuse, 2.2 times more likely to have decreased the number of their sex partners, 1.9 times more likely to have decreased the frequency of unprotected sex, 1.9 times more likely to have reduced their levels of alcohol use, and 2.3 times more likely to have decreased their crack use. These data support the importance of HIV testing and risk-reduction programmes for drug-involved women sex workers.