It is known that blood transfusion at the time of kidney transfplantation induces immune tolerance, leading to an increased rate of fixation of the transplanted kidney. A corollary of this phenomenon is the possibility that blood transfusion combined with surgical removal of a malignant tumor will promote recurrence and growth of the tumor. The prognoses of 51 patients after radical surgery for colon cancer of stage II or more (36 with blood transfusion and 15 without) and 51 patients after radical surgery for breast cancer (24 with blood transfusion and 27 without) were analyzed by the Kaplan-Meier method in relation to the presence or absence of combined blood transfusion. Among the patients with colon cancer, the survival rate for those given blood transfusion was obviously lower than that for those who were not (p<0.0001). Similarly, the recurrence rate of breast cancer was significantly higher in patients with blood transfusion than in those without. The prognosis was also clearly worse in colon cancer patients given blood transfusion than in corresponding patients who were not. A study of the possible relationship between the quantity of transfused blood and the prognosis revealed no correlation. These findings led us to speculate that blood transfusion before and after surgery serves as a factor for worsening the prognosis of cases of colon cancer and breast cancer, and that the presence of transfusion itself is important, regardless of the quantity of transfused blood.