Intravenous drug users are the second largest group to develop the acquired immunodeficiency syndrome, and they are the primary source for heterosexual and perinatal transmission in the United States and Europe. Understanding long-term trends in the spread of human immunodeficiency virus among intravenous drug users is critical to controlling the acquired immunodeficiency syndrome epidemic. Acquired immunodeficiency syndrome surveillance data and seroprevalence studies of drug treatment program entrants are used to trace seroprevalence trends among intravenous drug users in the borough of Manhattan. The virus entered this drug-using group during the mid-1970s and spread rapidly in 1979 through 1983. From 1984 through 1987, the seroprevalence rate stabilized between 55% and 60%--well below hepatitis B seroprevalence rates. This relatively constant rate is attributed to new infections, new seronegative persons beginning drug injection, seropositive persons leaving drug injection, and increasing conscious risk reduction.