To estimate the recent course of the human immunodeficiency virus type 1 (HIV) epidemic among men within birth cohorts, ethnic groups, and HIV-risk groups in Los Angeles County, backcalculation methods were combined with log-linear models and census data to reconstruct HIV incidence in subgroups from AIDS surveillance data. Results were compared with directly measured HIV seroprevalence in public sexually transmitted disease (STD) clinics in Los Angeles. Models of HIV incidence indicate that the initial epidemic pattern among men who have sex with men, including a decline in incidence since the mid-1980s, does not apply to all post-1960 birth cohorts. Later peaks were observed in younger birth cohorts and among injection drug users, especially among African-American men, with no evidence of a peak before the 1990s among men born after 1960. Our results indicate that HIV continued to spread near peak rates into the 1990s among younger birth cohorts, especially among young African-American men who have sex with men. Because of the lengthy incubation period from HIV infection to AIDS incidence, our results imply that the AIDS epidemic has not yet peaked in these cohorts and may continue to grow through the present decade in several subgroups. The large variation in HIV incidence and prevalence across birth cohorts and other subgroups needs to be addressed in future community intervention plans.