Thirtyeight patients with suspected thrombosis in the legs were investigated scintigraphically using 99mTc-tripolyphosphate. The ratio between the activity in the leg with suspected thrombosis and the other leg was calculated. The results were compared with those obtained with phlebography. A pathologically high activity ratio was found in all but one of the 18 patients in whom phlebography showed a thrombus. The ratio was normal in 13 of the 20 patients with normal phlebographic findings. The possible use of the method described as a postoperative screening test is discussed.