A histopathological study of 703 surgical specimens from patients with adenocarcinoma of the rectum revealed invasion of veins by primary growth in almost 52 per cent. Follow-up studies on the patients showed that the corrected 5-year survival rate was significantly worse and liver metastases developed more frequently when venous invasion was present. Invasion of extramural veins was particularly significant whereas spread confined to intramural veins was less important. Invasion of large (thick-walled) veins was of greater consequence than invasion of small (thin-walled) veins and spread into thick-walled extramural veins had the greatest adverse influence of all. Venous spread of tumour takes place in parallel with local spread as measured by the Dukes' stage but exerts an influence on prognosis independent of the Dukes' stage. Similarly, vein invasion parallels the number of lymph node metastases but appears to exert an independent influence on prognosis. Observation of venous spread provides a precise assessment of the likely behaviour of rectal carcinoma and supplements, but does not replace indices such as the Dukes' stage or the number of lymph node metastases in routine use. The implications for surgical technique and management are discussed.