The transfusion-induced immune suppression that prolongs kidney graft survival for transplant patients may be detrimental to patients with malignancies. We studied the relationship of blood transfusion to the disease-free survival of 169 patients with operable breast cancer who had undergone mastectomy with axillary dissection at Mount Sinai Hospital between 1964 and 1972. The cumulative 5-year disease-free survival rate for patients who had received transfusions was 51% compared with 65% for patients who had not received blood (p = 0.0210). The two groups of patients were comparable in age, stage, discharge hemoglobin values, proportion of radical mastectomies, and duration of follow-up. Admission hemoglobin values were lower and operative blood loss was higher among patients who had received transfusions, and significant survival differences were noted in relation to operative blood loss: 69% of women with estimated blood loss less than the mean of 370 ml were free of disease at 5 years compared with 50% of women with higher intraoperative losses (p = 0.0279). However, the first year after operation the association of survival with transfusion was highly significant (77% for those who had received transfusions, 94% for those who had not, p = 0.0096), whereas survival rates in relation to operative blood loss differed by only 7% during the same interval (p = 0.1182). These results indicate that perioperative blood transfusion may be a significant prognostic factor for patients undergoing mastectomy for operable breast cancer.