BACKGROUND The effect of acute kidney injury (AKI) on anemia has been well-documented. However, the effect of 'preexisting' anemia on AKI has been less addressed. The aims of the present study were to investigate (1) the association between anemia at hospital admission and AKI occurrence, and (2) the effect of 'preexisting' anemia on the clinical outcomes of AKI. METHODS A retrospective cohort study was undertaken among patients aged > or =17 years who were admitted to our hospital during the year 2006 (n = 34,802). Anemia at hospital admission and AKI occurrences were determined using the WHO definition and the RIFLE criteria, respectively. A subgroup of patients with an estimated glomerular filtration rate > or =60 ml/min/1.73 m(2) was analyzed separately to control for the effect of chronic kidney disease on anemia. RESULTS The cumulative incidence of AKI was 11.2% in anemic patients at hospital admission, compared to 5.5% in nonanemic subjects. The association between anemia at admission and AKI occurrence remained statistically significant after controlling for potential confounders (odds ratio 1.5, 95% CI 1.4-1.6). In addition, an association between anemia at hospital admission and clinical outcomes of AKI was observed. CONCLUSIONS Anemia at hospital admission should be recognized as a potential risk factor for in-hospital AKI occurrence.