Perioperative blood transfusion (PBT) has proved to increase the risk of sepsis after surgery. The hypothesis that PBT also might increase the occurrence of leak of rectal anastomoses has been tested. The data of 94 patients undergone low anterior resection for rectal cancer were reviewed. Thirty-nine of them (41%) received one or more packed red cells (PRC) units perioperatively. The total dehiscence rate was 14% (13 cases), and it was significantly lower in not-transfused (4%) compared to transfused patients (28%) (p < 0.005). The risk of dehiscence was increased by six-fold if up to two, and by fifteen-fold if more than two PRC units were given. These findings suggest that leakage of rectal anastomoses should be considered among the detrimental effects of PBT. The risk of dehiscence increases with the number of units transfused. A judicious administration of blood might contribute to reduce this complication.