Background: The effect of blood transfusion on tumor growth is controversial. Under experimental conditions, even similar animal models can give varied results. This study was undertaken to characterize the nature of the effect of blood transfusion on tumor growth. Methods: Sixty-five Fischer 344 rats subcutaneously implanted with a methylcholanthrene-induced sarcoma were studied with additive blood transfusion at 1% tumor burden in two separate experiments. In experiment 1, the effects of syngeneic fresh whole blood transfusion (5, 10, and 15 ml/kg) and allogeneic (5 ml/kg) were tested. To determine if stored blood influenced the results, experiment 2 was performed with syngeneic blood transfusion (15 ml/kg) and allogeneic blood transfusion at 5 ml/kg. Tumor dimensions were determined daily by external measurement, and tumor weight and growth rate were calculated. Results: No significant differences in final tumor weights or tumor growth rates were found in transfused rats compared with controls. This held true for syngeneic blood transfusion regardless of dose, allogeneic blood transfusion, and regardless of whether the blood was fresh or stored. Conclusions: Additive blood transfusion does not affect tumor growth in this animal model. This finding, together with the general inconclusiveness in the reported literature on this topic, speaks against a dominant role for the effect of blood transfusion on tumor behavior. © 1996 The Society of Surgical Oncology, Inc.