Since human immunodeficiency virus (HIV) is predominantly sexually transmitted, serologic surveys for HIV infection in sexually transmitted disease (STD) clinics provide sentinel observations regarding HIV epidemiology. Over the past decade, 17,207 systematically collected sera from patients attending Baltimore STD clinics were analyzed. From 1979 through 1989, HIV seroprevalence rose from 0.23% to 5.35%, increasing significantly in both men and women (P less than .001). Due to a marked increase in HIV infection among women during the mid-1980s, the male-to-female ratio of HIV infection declined from 16:1 in 1979-1982 to 1.0 in 1988-1989. HIV seroprevalence increased significantly (P less than .001) in all age groups, with the greatest increase among teenagers, rising from 0.18% in 1979-1983 to 2.1% in 1987-1989 (P less than .001). Although HIV seroprevalence was higher among whites than blacks during the early 1980s, it increased in blacks subsequently (P less than .001), eventually resulting in a greater rate among black than white clinic patients (P less than .01). These data reflect the evolution of the HIV epidemic in US inner cities. HIV prevalence has increased greater than 20-fold, with recent increases being most marked among women, teenagers, and blacks. Additional resources will undoubtedly be required to support further intensive behavioral and educational programs targeted at adolescents and inner-city minorities.