OBJECTIVE: To assess and estimate trends in HIV, sexually transmitted infections (STIs), and sexual behavior among men who have sex with men (MSM) in Lima, Peru. DESIGN: Second-generation HIV sentinel surveillance surveys conducted in 1996, 1998, 2000, and 2002. METHODS: Adult men reporting sex with at least 1 man during the previous year were eligible to participate. Sexual behavior and serum HIV-1 and syphilis antibodies were assessed. HIV seroincidence was estimated by a sensitive/less-sensitive enzyme immunoassay strategy. Rectal and pharyngeal swabs for Neisseria gonorrhoeae culture and a first-void urine sample for urethral leukocytes for presumptive diagnosis of urethritis were obtained. Herpes simplex virus 2 (HSV-2) antibodies were measured in 2002. RESULTS: Although HIV prevalence increased from 18.5% to 22.3% from 1996 through 2002, bacterial prevalence declined significantly for syphilis (16.0% to 12.4%), early syphilis (8.6% to 3.4%), and rectal gonorrhea (5.1% to 0.2%). High HIV seroincidence was estimated, with the lowest (4.8%) incidence in 1998. In 2002, HSV-2 seroprevalence was 51.0%. After adjustment for age, education, and self-reported sexual identity, our data suggest that a yearly increase by 6% in the prevalence of HIV occurred among MSM in Lima, with a corresponding decline in syphilis (by 9%), early syphilis (by 18%), and rectal gonorrhea (by 64%). Condom use during last sexual intercourse increased by 26% each year with the most recent male steady partner and, among non-sex workers, by 11% with the most recent casual partner. CONCLUSIONS: HIV continued to spread among MSM in Lima even when a decline in bacterial STIs and increase in condom use were estimated to occur. Intensification of medical and behavior prevention interventions is warranted for MSM in Peru. © 2007 Lippincott Williams & Wilkins, Inc.