To investigate whether markers of haemostasis activity increased during preoperative radiotherapy and whether postoperative marker levels were increased in irradiated rectal cancer patients when compared with nonirradiated rectal and colon cancer patients. In 45 rectal cancer patients, we measured plasma levels of prothrombin fragment 1 + 2 (F1 + 2), thrombin-antithrombin complex, and D-dimer during radiotherapy. Postoperative levels of F1 + 2, thrombin-antithrombin complex, and D-dimer in irradiated patients were compared with postoperative levels in 123 nonirradiated colon and rectal cancer patients. A small oscillation in F1 + 2 levels was observed during radiotherapy among long-term low-intensity radiotherapy recipients. Postoperative levels of F1 + 2 and D-dimer were significantly higher among patients who received short-term high-intensity radiotherapy. This study provided no evidence for activation of the haemostatic system during preoperative radiotherapy in patients with rectal cancer. Some evidence was provided for increased postoperative haemostatic activity among rectal cancer patients who received short-term high-intensity radiotherapy, when compared with patients who received long-term low-intensity radiotherapy, and nonirradiated patients.