['Current blood transfusion standards in Canada and the United States permit transfusion of ABO-nonidentical platelets when ABO-identical platelets are not available. This practice increases the availability of platelets, a component in chronic shortage in Ontario, Canada because of the 5-day shelf-life. The impact of transfusing ABO-nonidentical platelets on patient outcomes is unknown. STUDY DESIGN AND METHODS. A retrospective review of 1721 patients who had cardiovascular surgery between November 1989 and December 1999 and who had also received a platelet transfusion perioperatively was conducted. The impact of platelet and plasma incompatibility on clinical outcomes was analyzed.', 'The analysis included 1691 patients who were divided into two groups according to the compatibility of the first platelet transfusion received: ABO-identical platelet transfusion (n = 1008) and ABO-nonidentical platelet transfusion (n = 683). The only difference in baseline characteristics between the two groups was that there were more urgent cases in the ABO-identical platelet transfusion group (p = 0.04). There were no significant differences in mortality at 30 days (10% for both groups, p = NS) or in postoperative length of stay (median, 7.0 days for both groups, p = NS). No significant differences were found with respect to the use of blood components, indices of bleeding, incidence of infection, or platelet CCIs.'] Is transfusion of ABO-nonidentical platelets associated with adverse clinical outcomes in cardiovascular surgery patients?