The incidence of deep venous thrombosis of the legs (DVT) was studied in 119 critically ill patients by 125I-labeled fibrinogen scanning; the efficacy of low-dose heparin prophylaxis was assessed in a randomized, double-blind study. DVT occurred in 29% of control patients and in 13% of patients receiving heparin 5000 U subcutaneously twice daily. DVT was found mainly in men and was associated with circulatory impairment, respiratory failure and recent vascular or cancer surgery. In a comparison study of medical patients, DVT occurred in 10% untreated and 2% treated. In conclusion, the critically ill are at high risk of venous thromboembolism and low-dose prophylaxis is warranted in those who have no hemostatic impairment.