Risk factors for anaemia and cumulative incidence of anaemia were examined in a longitudinal cohort study of 622 injection drug users (IDUs) (8885 visits) in Baltimore, Maryland, from 1988 to 2000. At enrollment, 19.6% subjects were anaemic (16.1% of men and 30.5% of women, P<0.0001) and 8.4% had microcytic anaemia (6.7% of men and 14.0% of women, P=0.006). Cumulative incidence of anaemia was 82.2% (87.9% of men and 100% of women, P<0.0001) during a median of 7.5 years follow-up. Factors associated with anaemia included age (per 5 year increase, odds ratio (OR)=1.22; 95% confidence interval (CI): 1.10, 1.36), female gender (OR=1.62; 95% CI: 1.16, 2.27), CD4+ lymphocyte count <200 cells/microl (OR 1.85; 95% CI: 1.52, 2.24), weight loss (OR 1.55; 95% CI: 1.26, 1.91), oral thrush (OR 1.53; 95% CI: 1.21, 1.94), Mycobacterium avium complex infection (OR 1.30; 95% CI: 1.04, 1.64), and zidovudine use (OR 1.24; 95% CI: 1.04, 1.48). Higher body mass index (OR 0.92; 95% CI: 0.88, 0.95) and marijuana use (OR 0.75; 95% CI: 0.61, 0.92) were associated with a lower risk of anaemia. The cumulative incidence of anaemia is high among IDUs, and women are at highest risk of anaemia.