The Bronx, a borough of New York City with 1.16 million people, has a distinctive pattern of prevalence and distribution of acquired immunodeficiency syndrome (AIDS), i.e., 62.2% of AIDS patients are intravenous drug users, 20.3% are female, 87.3% are black or Hispanic, and 4.5% are children under age 13 years. Local data on reported AIDS cases by risk factors, age, and sex are combined with local indices of the intravenous drug use population to estimate numbers of intravenous drug users. The Bronx is estimated to have 40,400 intravenous drug users (range, 28,080-52,800), 78% of whom fall into the age group 25-44 years. On the basis of local serosurveys, 45-55% are considered to be human immunodeficiency virus (HIV)-positive. With Bronx population census data as the denominator, minimum rates of HIV seroprevalence are calculated for all Bronx males and females aged 25-44 years, a group comprising 76% of the AIDS cases in the Bronx through February 1, 1987. These data produce a population seroprevalence range for those aged 25-44 years of 5.4-12.5% for all Bronx males and 1.4-3.3% for females. For the South Bronx, with 66% of all intravenous drug users and 38% of the population, these age-specific HIV prevalence estimates are 9.4-21.6% for males and 2.4-5.5% for females.