As AIDS cases among US women continue to increase, a better understanding of women's behavioural risk patterns is needed to inform intervention efforts. Data were from 2,945 women drug injectors and crack users. Statistical analyses compared sociodemographic variables, lifetime behavioural risk patterns, HIV sero-prevalence and history of sexually transmitted diseases, and determined predictors of HIV infection separately in 16 low and four high sero-prevalence sites. Based on risk patterns, four behaviourally-defined sub-groups were constructed, and rates of HIV sero-prevalence were compared. In comparisons between low and high sero-prevalence sites, there were significant differences on most variables examined, and in the relative importance of the sociodemographic characteristics and risk patterns predicting HIV. Drug injection and sex exchange were each independent, significant, behavioural predictors of infection, with no significant difference between the odds ratios attributed to each predictor. HIV sero-prevalence was significantly different among four sub-groups. Interventions must be tailored to address observed differences among women in low and high sero-prevalence sites. Injection drug use and exchanging sex each play a major role in the transmission of HIV infection to US women. Prevention efforts targeted at women should address differences in behavioural risk patterns. Aggressive and innovative interventions are needed for women who exchange sex. AIDS research must investigate how socioeconomic factors impact women's risk for HIV infection.