Background: We assessed prevalences of seven sexually transmitted infections (STIs) in Peru, stratified by risk behaviours, to help to define care and prevention priorities. Methods: In a 2002 household-based survey of the general population, we enrolled randomly selected 18-29-year-old residents of 24 cities with populations greater than 50 000 people. We then surveyed female sex workers (FSWs) in these cities. We gathered data for sexual behaviour; vaginal specimens or urine for nucleic acid amplification tests for Neisseria gonorrhoeae, Chlamydia trachomatis, and Trichomonas vaginalis; and blood for serological tests for syphilis, HIV, and (in subsamples) herpes simplex virus 2 (HSV2) and human T-lymphotropic virus. This study is a registered component of the PREVEN trial, number ISRCTN43722548. Findings: 15 261 individuals from the general population and 4485 FSWs agreed to participate in our survey. Overall prevalence of infection with HSV2, weighted for city size, was 13·5% in men, 13·6% in women, and 60·6% in FSWs (all values in FSWs standardised to age composition of women in the general population). The prevalence of C trachomatis infection was 4·2% in men, 6·5% in women, and 16·4% in FSWs; of T vaginalis infection was 0·3% in men, 4·9% in women, and 7·9% in FSWs; and of syphilis was 0·5% in men, 0·4% in women, and 0·8% in FSWs. N gonorrhoeae infection had a prevalence of 0·1% in men and women, and of 1·6% in FSWs. Prevalence of HIV infection was 0·5% in men and FSWs, and 0·1% in women. Four (0·3%) of 1535 specimens were positive for human T-lymphotropic virus 1. In men, 65·0% of infections with HIV, 71·5% of N gonorrhoeae, and 41·4% of HSV2 and 60·9% of cases of syphilis were in the 13·3% who had sex with men or unprotected sex with FSWs in the past year. In women from the general population, 66·7% of infections with HIV and 16·7% of cases of syphilis were accounted for by the 4·4% who had been paid for sex by any of their past three partners. Interpretation: Defining of high-risk groups could guide targeting of interventions for communicable diseases-including STIs-in the general Peruvian population. Funding: Wellcome Trust-Burroughs Wellcome Fund Infectious Disease Initiative and US National Institutes of Health. © 2012 Elsevier Ltd.