Objective: The storage time of allogeneic red blood cells (RBCs) has been linked with the risk of severe postoperative infections following cardiac surgery. However, existing data are sparse and inconsistent. We therefore examined the association between the age of transfused RBCs and development of severe postoperative infection following coronary artery bypass grafting (CABG) in a large population-based cohort study. Methods: The study included patients undergoing CABG with or without concomitant cardiac surgery between June 2003 and July 2008 in the North and Central Denmark regions. Data on demography, perioperative variables, allogeneic blood transfusion and severe postoperative infections (deep sternal wound infection, bacteremia or septicemia) were retrieved from medical databases and medical records. We used logistic regression analyses to compute the crude and adjusted odds ratios (ORs) with 95% confidence intervals (CIs) for the association between storage time of transfused RBCs and the risk of severe infection. Results: A total of 4240 patients were included in the final analyses, and 1748 of these patients (41%) were transfused with RBCs. Among transfused patients, 953 were exclusively transfused with RBC stored for /=14 days. Severe infection was identified in 165 patients (3.9%). The adjusted ORs for severe infection among all transfused patients and patients transfused with RBCs stored exclusively for either /=14 days were 1.6 (95% CI: 0.9-2.8), 1.1 (95% CI: 0.6-2.1), and 2.3 (95% CI: 1.2-4.2), respectively, when compared with non-transfused patients. There was a dose-response relationship between the number of transfused RBC units and the risk of severe infection among patients exclusively transfused with RBCs stored for >/=14 days. Conclusion: Although the risk of possible confounding could not be eliminated entirely in this observational study, the findings add further support for the hypothesis that storage time of RBCs is positively associated with the risk of transfusion-related severe postoperative infection in patients undergoing CABG.