THE HE DIAGNOSIS of deep venous thrombosis based on clinical findings remains relatively imprecise.1 In one series,2 80% of patients with fatal pulmonary emboli gave no prior clinical evidence of thromboembolic disease. Although phlebography has proved a most valuable diagnostic tool for determining the presence of thrombi within the deep veins of the extremities,3 it has the disadvantages that this technique requires transportation of the patient to an x-ray suite and a trained team and occasionally evokes significant pain. Hobbs and Davies,4 combining the administration of fibrinogen labeled with radioactive iodine (131I) and counting with an external scintillation counter, were able to demonstrate the presence of thrombi by detecting the concentrated radioisotope in clots in the superficial saphenous veins of rabbits. The thrombi had been induced subsequent to the injection of the radioactive material and the authors presented evidence that when radioactive fibrinogen is circulating