The doppler ultrasound probe was used to study 138 patients after major surgery to assess the incidence of iliofemoral thrombosis in the early postoperative period. The patients were simultaneously studied with the 125-I-fibrinogen uptake test. The accuracy of the ultrasonic technique was assessed concurrently by comparing its results with those of ascending phlebography in a separate group of patients. The ultrasound probe detected thrombus proximal to the calf but was inaccurate in locating its exact site. Of the 138 patients studied postoperatively 28 developed calf vein thrombosis detected istopically, but there was no evidence of iliofemoral thrombosis in any patient. Thus, spontaneous iliofemoral thrombosis in the early postoperative period is rare.