The objective was 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. 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. 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. 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. (authors)