Objective-To assess the effects of perioperative blood transfusion on cancer related survival and infective complications after radical operations for colorectal cancer. Design-Retrospective study. Setting-District hospital in Sweden. Subjects-217 patients who fulfilled the criteria for inclusion, out of 392 consecutive patients operated on for colorectal cancer between 1975 and 1979. Main outcome measures-Morbidity and cancer related mortality depending on whether blood was transfused and, if so, how much. Results-Dukes' stage (p < 0.001), rectal tumours (p < 0.05) and the number of units transfused (p < 0.05) were significantly associated with cancer related mortality. Patients with rectal cancer transfused with 1-2 units had significantly better survival than those transfused with more than 4 units (p < 0.05), but this was not the case for colonic tumours. There was no significant association between blood transfusion and the incidence of infective complications. Conclusion-Though there seems to be an association between the number of units of blood transfused and cancer related survival in patients with rectal cancer, this does not necessarily imply causation. We recommend that until this is clarified by large, prospective investigations, autologous blood should be used whenever possible, and unnecessary blood transfusion should be avoided.