A total of 846 surgical and medical patients have been screened by the 125I-fibrinogen test (I.F.T.). Deep vein thrombosis (D.V.T.) was detected in 174 (21%). In 49% of positives the condition was bilateral and in 33% the D.V.T. involved popliteal veins or above. In 14 patients pulmonary embolism was diagnosed by lung scan or at autopsy. In one massive embolism was the cause of death. All 14 had positive I.F.T’s. ; 10 bilateral and 10 with both calf and thigh vein thrombus. The relationship between positive I.F.T. and iliofemoral D.V.T. was studied with bilateral phlebography in 60 patients. In no case was isolated iliofemoral D.V.T found. We confirm the value of the I.F.T. as a screening test for potential embolism and observe that embolism from isolated iliofemoral D.V.T. is extremely rare.