Objective: To determine the extent of the HIV-1 epidemic in Peru. Design: Part of a national serosurvey in Peru. Methods: Between January 1986 and December 1990, 140 976 serum samples were tested for HIV-1 antibody. Results: HIV-1 antibody was found in a high percentage of serum samples provided by 4300 homosexual men (26%), 2204 male sexually transmitted disease patients (10%), 145 drug users (13%), 269 hemophiliacs (10%), and 146 unlicensed female prostitutes (10%). In addition, the prevalence of HIV-1 infection increased substantially among these groups between the beginning and end of the survey period. A low but rising prevalence of HIV-1 antibody was found during this period among serum samples provided by 83 526 blood donors and 11 101 military personnel: total period prevalence, 0.25 and 0.32%, respectively. Conclusion: These data indicate that HIV-1 infection is epidemic in Peru among groups at high risk of sexually and parenterally transmitted diseases, and that the risk of infection appears to be low but possibly increasing among the general population.