Street-based female sex workers constitute a vulnerable population for HIV, as they are often enmeshed in chronic patterns of substance use, sexual risk, homelessness, and violent victimization. This study examined the specific contributions of victimization history and abuse-related traumagenic factors to mental health functioning and sexual risk behaviors, while considering the impact of environmental risk factors as well. Using targeted sampling strategies, we enrolled 562 Miami-based female sex workers into an intervention trial testing the relative effectiveness of two alternative case management conditions in establishing linkages with health services and reducing risk for HIV. Lifetime prevalence of abuse was extremely elevated at 88%. Nearly half reported abuse before the age of 18, while 34% reported violent encounters with "dates" or clients in the past 90 days. Serious mental illness (SMI) was quite common, with 74% reporting severe symptoms of depression, anxiety, or traumatic stress. For those with histories of abuse, SMI appeared to mediate the association between abuse-related trauma and unprotected sex behaviors. Mental health treatment would appear to be an important component of effective HIV prevention among this vulnerable group, and should form part of a compendium of services offered to female sex workers.