BACKGROUND: A randomized, double-blind trial is reported of the clinical efficacy of red blood cells (RBCs) treated for pathogen inactivation with S-303, a synthetic labile alkylating agent. STUDY DESIGN AND METHODS: Patients undergoing complex cardiac surgeries were randomly assigned to receive either S-303-treated (test) or conventional (control) RBC transfusion during surgery and for 6 days thereafter. Efficacy was evaluated by comparing the occurrence of a composite primary endpoint of treatment-related morbidity (myocardial infarction and renal failure) and mortality. RESULTS: Two-hundred twenty-three patients were randomly assigned and 148 patients who received transfusions (74 with S-303-treated RBCs and 74 with control RBCs) were evaluable. The incidence of the primary endpoint was equivalent between the two groups (22 and 21% in the S-303-treated and control RBC groups, respectively). Secondary endpoints, including hemoglobin increment (mean, 1.4 vs. 1.5 g/dL), number of RBC transfusions (mean, 4.4 vs. 3.8 units), and other blood product support, were also comparable. The adverse event profile was similar between groups; however, patients who received S-303 RBCs were significantly more likely to develop constipation and less likely to suffer supraventricular extrasystoles. Four patients (2 test and 2 control) demonstrated positive indirect antiglobulin tests with reactivity for S-303 RBCs at one or more time points before or after transfusion, without evidence of hemolysis. CONCLUSION: S-303-treated and conventional RBCs were equivalent with respect to clinical efficacy and safety in supporting the transfusion needs of cardiac surgery patients. Investigations are under way to ascertain the significance of S-303 RBC antibodies and to prevent their occurrence.