A group of 230 patients undergoing elective colorectal surgery was analyzed for the presence of deep venous thrombosis (DVT). Prophylaxis against DVT was practiced with low-dose heparin (either 5000 IU every eight hours, or 5000 IU every 12 hours for seven days) in 199 patients. Prevention of infection was attempted with preopetive administration of Enterobiotic^® in 155 patients and of Vibramycin^® in patiens. DVT was diagnosed in 46 patients. The frequency of DVT did not differ significantly between patients who underwent resections of the colon and those who underwent rectal surgery. DVT was diagnosed in 27 of the 73 infected patients, which was significantly higher than the incidence of 19 with DVT among the 157 uninfected patients. The frequency of DVT among patients in the two heparin regimens was 15 and 17 per cent respectively, which was significantly lower with untreated patients. No lethal pulmonary embolism was found and no patient showed clinical signs of embolism. It is assumed that measures aimed at reducing postoperative infection, combined with low-dose heparin, will reduce the incidence of postoperative DVT after colorectal surgery.