Sexually transmitted infections (STIs) have shown to enhance the transmission of human immunodeficiency virus (HIV) and to be more common among female commercial sex workers (FSWs). A cross-sectional study was conducted among 625 FSWs in six cities of Argentina in 2000-2002. The seroprevalence of HIV, hepatitis B virus (HBV), hepatitis C virus (HCV), human T-cell lymphotropic virus type I/II, and syphilis was 3.2%, 14.4%, 4.3%, 1.6%, and 45.7%, respectively. Syphilis was associated with older age (≥ 30 years, adjusted odds ratio [AOR] = 2.6 to 4.9), ≥ 10 years in sex work (AOR = 2.2), use of illegal drugs (AOR = 2.1), and a prior history of an STI (AOR = 3.0). HBV and syphilis was the most common co-infection in 44 (7.5%) subjects. FSWs in Argentina are exposed to HIV and other STIs due to high-risk sexual and illegal drug use behavior. Renewed efforts are necessary to intervene effectively in this high-risk population. Copyright © 2006 by The American Society of Tropical Medicine and Hygiene.