No adequate data exist on patterns of injection drug use (IDU) prevalence over time within racial/ethnic groups in U.S. geographic areas. The absence of such prevalence data limits our understanding of the causes and consequences of IDU and hampers planning efforts for IDU-related interventions. Here, we (1) describe a method of estimating IDU prevalence among non-Hispanic Black and non-Hispanic White adult residents of 95 large U.S. metropolitan statistical areas (MSAs) annually over an 11-year period (1992--2002); (2) validate the resulting prevalence estimates; and (3) document temporal trends in these prevalence estimates. IDU prevalence estimates for Black adults were calculated in several steps: we (1) created estimates of the proportion of injectors who were Black in each MSA and year by analyzing databases documenting injectors' encounters with the healthcare system; (2) multiplied the resulting proportions by previously calculated estimates of the total number of injectors in each MSA and year (Brady et al., 2008); (3) divided the result by the number of Black adults living in each MSA each year; and (4) validated the resulting estimates by correlating them cross-sectionally with theoretically related constructs (Black- and White-specific prevalences of drug-related mortality and of mortality from hepatitis C). We used parallel methods to estimate and validate White IDU prevalence. We analyzed trends in the resulting racial/ethnic-specific IDU prevalence estimates using measures of central tendency and hierarchical linear models (HLM). Black IDU prevalence declined from a median of 279 injectors per 10,000 adults in 1992 to 156 injectors per 10,000 adults in 2002. IDU prevalence for White adults remained relatively flat over time (median values ranged between 86 and 97 injectors per 10,000 adults). HLM analyses described similar trends and suggest that declines in Black IDU prevalence decelerated over time. Both sets of IDU estimates correlated cross-sectionally adequately with validators, suggesting that they have acceptable convergent validity (range for Black IDU prevalence validation: 0.27 < r < 0.61; range for White IDU prevalence: 0.38 < r < 0.80). These data give insight, for the first time, into IDU prevalence trends among Black adults and White adults in large U.S. MSAs. The decline seen here for Black adults may partially explain recent reductions in newly reported cases of IDU-related HIV evident in surveillance data on this population. Declining Black IDU prevalence may have been produced by (1) high AIDS-related mortality rates among Black injectors in the 1990s, rates lowered by the advent of HAART; (2) reduced IDU incidence among Black drug users; and/or (3) MSA-level social processes (e.g., diminishing residential segregation). The stability of IDU prevalence among White adults between 1992 and 2002 may be a function of lower AIDS-related mortality rates in this population; relative stability (and perhaps increases in some MSAs) in initiating IDU among White drug users; and social processes. Future research should investigate the extent to which these racial/ethnic-specific IDU prevalence trends (1) explain, and are explained by, recent trends in IDU-related health outcomes, and (2) are determined by MSA-level social processes.