To monitor the trend of HIV-1 seroprevalence, 2,194 newly admitted injecting drug users (IDUs) in a methadone maintenance program in New York City were tested from June 1988 through June 1992. This was a blinded study and unlabeled blood specimens were collected during the required physical examination. There was a significant linear trend (p = .01) toward decline in HIV-1 seropositivity by year of admission as follows: 47.0% in 1988, 43.4% in 1989, 43.2% in 1990, 39.8% in 1991, and 39.1% in 1992. From January 1988 through December 1992, admissions data that included the newly admitted IDUs showed a significant decrease (p < .0002) in self-reported current needle sharing from 11.3% to 5.4%. These two data sources suggest that a continued decline in HIV-1 seroprevalence among new admissions may be related to decreased needle sharing. A volunteer cohort of 235 HIV-1 seronegative IDUs was also enrolled from April 1990 through June 1991 in a study of HIV-1 seroconversion. Follow-up data were available on 177 patients for up to 12 months. For a total observation time of 155 person-years, there were two conversions (1.3 conversions per 100 person-years). A continued decline in HIV-1 seroprevalence among new admissions and a low HIV-1 seroconversion rate among existing patients have contributed to a lower overall HIV-1 prevalence in clinic patients of about 40%. This stands in contrast to 1985-86 estimates of HIV-1 seroprevalence that plateaued at 60%.