National unlinked sentinel surveillance data were used to describe trends in prevalent human immunodeficiency virus infection among injection drug users entering drug treatment programs in the United States from 1988 through 1993. During this 6-year period, unlinked testing was performed on 70,882 specimens from injection drug users at 60 sentinel sites. The annual change in seroprevalence was estimated for each site by odds ratios obtained from logistic regression models fit within site-specific age and race/ethnicity subgroups. Overall trends for age and race/ethnicity subgroups across sites were described by summary odds ratios calculated using the inverse variance method. A decrease was observed among younger (age less than 30 years) whites both in areas with high (10% or higher) and low (less than 10%) prevalence, although this decrease was significant only in high-prevalence areas (odds ratio = 0.90, 95% confidence interval 0.81-0.99). Seroprevalence also decreased among older whites in high-prevalence areas, although this decrease was not significant (odds ratio = 0.95, 95% confidence interval 0.89-1.00). Seroprevalence remained stable among all other age and race/ethnicity subgroups. Stable seroprevalence among the dynamic population of injection drug users entering treatment suggests continued transmission among these individuals in both high- and low-prevalence areas of the United States.