A retrospective study of 192 patients operated on by three members of the Edward Wilson Colon and Rectum Unit, Sydney Hospital, was carried out. All operations performed involved dissection within the pelvis. Prophylactic administration of low-dose heparin was used for 71 of these patients. The incidences of clinical thromboembolic disease were 7 per cent in both the group receiving heparin and the other group. In slightly more than half of the patients, pulmonary emboli occurred in the absence of peripheral deep venous thrombosis. It is suggested that the source of these emboli was thrombosis arising in the pelvic veins.