OBJECTIVES: The objectives of this study were to determine the prevalence of sexually transmitted infection (STIs) in heterosexual couples and the sexual behaviors associated with their acquisition. GOAL: The goal of this study was to better direct educational efforts to decrease STI among heterosexual couples in Lima, Peru. STUDY DESIGN: We conducted a case-control study in 195 heterosexual couples without HIV infection who attended 2 sexually transmitted disease clinics in Lima, Peru. A case was defined as an individual with one or more newly diagnosed STIs such as gonorrhea, chlamydia, trichomoniasis, herpes simplex, syphilis, mycoplasma, or ureaplasma. RESULTS: Thirty-three percent of individuals (41 men and 89 women) had at least one STI and 26 couples (13%) had the same STI detected. Men who have sex with men (MSM) accounted for 13% of all men, had higher rate of STIs and higher risk behaviors than non-MSM. Ureaplasma infection was the most prevalent STI found in both men and women and was associated with oral sexual contact. In heterosexual pairs, condom use during anal sex occurred less than 10% of the time. CONCLUSIONS: The heterosexual couples in sexually transmitted disease clinics have high-risk behaviors and STIs are frequent. The educational effort concerning prevention of STIs requires higher effort. © Copyright 2007 American Sexually Transmitted Diseases Association.