A simplified technique for detecting phlebothrombosis of the legs is using radioiodinated fibrinogen. In 88 patients, the accuracy of the test was 92%, as determined by concurrent phlebography. The test was also applied in 784 patients in various high risk groups, wherein the incidence of phlebothrombosis was as follows: 469 elective surgical patients, 27.8%; 50 patients with hip fractures, 54%; 42 patients undergoing prostatectomy, 23.8%; 100 obstetric patients, 3%; 92 women undergoing major gynecological surgery, 18.8%; and 31 patients with proven myocardial infarction, 19%. The method was also useful in 82 patients thought to have established phlebothrombosis by clinical criteria. Disadvantages of the test include its inability to detect phlebothrombosis in the upper femoral and iliac regions, the possible risk of serum hepatitis (no cases seen in over 800 patients studied), and the number of personnel required if a large number of patients are to be screened. Its advantages are simplicity, accuracy, portability, and lack of trauma.