Objectives. This study examined the prevalences of antibodies to Treponema pallidum, Chlamydia trachomatis, and herpes simplex virus type 2 in a sample of Peruvian adults. Methods. Among adults seeking health certification in Lima, Peru, 600 were randomly selected to undergo interviews and serologic testing. Results. Men's reported mean lifetime number of partners (10.6) far exceeded women's (1.1), yet antibody to sexually transmitted infection pathogens among sexually experienced participants was 2.8 times more prevalent among women than among men. Among men, female sex workers accounted for 37% of recent partners, and only sex with female sex workers while using condoms less than half of the time was independently associated with antibody (odds ratio = 3.6, 95% confidence interval = 1.5, 8.8). Among women, number of partners was associated with any sexually transmitted infection antibody, while intercourse before 18 years of age was associated with C. trachomatis antibody. At every level of perceived risk, sexually transmitted infection antibody was more frequent among women. Conclusions. Men having unprotected sex with female sex workers had the greatest risk of acquiring infections and (by inference) of transmitting them to women.