The fibrinogen labeled with radioactive iodine (125I) uptake test (FUT) is an established clinical test at St. Thomas's Hospital, London. It is performed with the patient in the supine position, with use of the Negus technique, with a collimator aperture of 3 inches, and scintillation counter connected to a scaler and timer. In studying 102 patients with the FUT and phlebography, the FUT findings agreed with those of phlebography in 80% of the limbs. Thus, the FUT is an accurate method for diagnosing deep vein thrombosis between the junction of the middle and upper thirds of the thigh and the lower calf. The test is currently used in screening of patients considered to be at risk for thromboembolism.