A cross-sectional anonymous study of 261 STD (sexually transmitted diseases) outpatients and 288 outpatients from other hospital departments was conducted at four major city hospitals in Buenos Aires. High prevalence of human immunodeficiency virus (HIV) (14.5%) and syphilis (30.2%) was noted. Fifty-two persons were diagnosed with both HIV and syphilis. Of the 87 HIV cases observed, 52 (59.7%) were co-infected with syphilis. Stratified analysis by gender showed that the prevalence of HIV, syphilis and HIV/syphilis co-infection was significantly (p < 0.001) higher in men than women (HIV: 20.1% vs. 4.6%; syphilis: 39.3% vs. 17.4%; co-infection: 13.6% vs. 1.7%). Integrated HIV/STD intervention programs and more effective surveillance are required in Argentina.