Objective: Perioperative blood transfusions may adversely affect survival in patients with colorectal malignancy, although definite proof of a causal relationship has never been reported. Background: We report the long-term outcomes of a randomized controlled trial performed between 1986 and 1991 to compare the effects of allogeneic blood transfusions and an autologous blood transfusion program in colorectal cancer patients. Methods: All 475 randomized patients operated upon for colorectal cancer were tracked via a national computerized record-linkage system to investigate survival and cause of death. Kaplan-Meier survival curves were constructed and multivariate Cox regression analysis was performed to study 20 years' overall survival. Colorectal cancer-specific survival was analyzed over the 10-year time period after surgery. Results: The overall survival percentage at 20 years after surgery was worse in the autologous group (21%) compared to the allogeneic group (28%) (P = 0.041; log-rank test). Cox regression, allowing for tumor stage, age, and sex, resulted in a hazard ratio (autologous vs allogeneic group) for overall mortality of 1.24 (95% confidence interval 1.00-1.54; P = 0.051). Colorectal cancer-specific survival at 10 years for the whole study group was 48% and 60% for the autologous and allogeneic group, r Conclusions: At long-term follow-up colorectal cancer patients did not benefit from autologous transfusion compared with standard allogeneic transfusion. On the contrary, the overall and colorectal cancer-specific survival rates were worse in the patients in the autologous transfusion group.