OBJECTIVE To examine trends in acquired immunodeficiency syndrome (AIDS) risk behavior and human immunodeficiency virus (HIV) seroprevalence among injecting drug users (IDUs) in New York City from 1984 through 1992. METHODS Comparisons were made between two surveys of IDUs at the same hospital-based New York City drug abuse detoxification program: 141 IDUs in 1984 and 974 IDUs in 1990 through 1992. National Death Registry, New York City Health Department, and drug treatment program records were also used. METHODS Persons attending detoxification program randomly selected for participation. Eligibility was based on injection within previous 2 months; 99% acceptance rates were obtained. Participants in the 1984 and 1990 through 1992 surveys were 66% and 79% men, 21% and 19% white, 33% and 34% African American, and 45% and 46% Latin American, respectively. METHODS Community-based AIDS prevention programs, including underground syringe exchanges. METHODS Acquired immunodeficiency syndrome risk behaviors; HIV serostatus; CD4+ cell counts; death rates among 1984 subjects; and injection and intranasal routes of drug administration. RESULTS The HIV seroprevalence remained stable at slightly more than 50%. Mean CD4+ cell counts declined from 0.716 x 10(9)/L (716/microL) to 0.575 x 10(9)/L (P < .009). Annual death rate among 1984 subjects was 3%, with a significantly higher rate among HIV-seropositive subjects (relative risk, 2.57; 95% exact binomial confidence interval, 1.12 to 6.61). Large-scale declines were observed in AIDS risk behaviors, eg, use of potentially contaminated syringes declined from 51% to 7% of injections (P < .001). Recent additional risk reduction was associated with use of the underground syringe exchanges. Intranasal heroin use was the primary route of drug administration for 46% of heroin admissions to New York City drug treatment programs. CONCLUSIONS The HIV seroprevalence has remained stable among this population of New York City IDUs for almost a decade. Continuation of current trends should lead to further reduction in HIV transmission, although reversal of the trend to intranasal use could lead to substantially increased transmission.