Histopathological material from operation specimens of rectal adenocarcinoma was reviewed and invasion of veins identified in 51.9% of 703 cases. The extent of venous invasion, thickness of the walls of invaded veins and various other histological features were examined in detail. By follow-up studies recurrence rates, incidence of distant metastases and corrected 5-year survival rates were obtained; correlation with the histopathological results showed that invasion of extramural and thick-walled veins is associated with a poor prognosis independent of the degree of differentiation of the adenocarcinoma. Prognosis is not significantly related to the presence of necrosis of intravenous tumour and a clearly defined stroma in the intravenous growth all appear to exert a protective influence on patient survival. Whereas permeation of capillaries in vein walls, the presence of loose clumps of tumour cells in veins and direct contact between tumour cells and venous blood appear to adversely affect survival. Venous invasion is shown to be related to local invasiveness of rectal carcinoma. Suggested modes of venous spread and interaction with host tissues are proposed, with implications for general attitudes to the spread of cancer and metastasis.