Propagation of deep venous thrombosis identified by duplex ultrasonography. To investigate the efficacy of anticoagulation in preventing continuing thrombosis, we prospectively evaluated 24 patients with acute deep venous thrombosis using duplex ultrasonography. All patients were hospitalized with conclusive ultrasonic evidence of deep venous thrombosis identified in one of four levels: I, calf only; II, calf-popliteal; III, calf-popliteal-femoral; or IV, calf-popliteal-femoral-iliac. Duplex scans were obtained on admission and on three subsequent occasions during therapy. Progression of thrombosis was defined as advancement of thrombus to the more proximal venous level. Demographic data, symptoms, risk factors for deep venous thrombosis, physical findings, anticoagulation regimens, and hematologic variables were ascertained. Adequacy of anticoagulation was defined as elevation of baseline activated partial thromboplastin time by 150%. Nine patients (38%) had progression of thrombosis, and 15 (62%) had stable or improving duplex scans. Progression occurred as follows: I----II (2), I----III (2), II----III (1), and III----IV (4). Of the demographic and clinical variables examined, only smoking correlated with progression of thrombus (p = 0.04). Average heparin dose in the stable group was 1214 +/- 294 units/hr and 1122 +/- 248 units/hr in the group that progressed (p = 0.8): activated partial thromboplastin time was 45.6 +/- 7 seconds in the stable group and 49.8 +/- 9 seconds in the progression group (p = 0.7). Nine patients in the stable group had consistently adequate anticoagulation, whereas six did not; six in the progression group were consistently anticoagulated, and three were not. Two patients (one with stable thrombus and one with progressive thrombus) suffered nonfatal pulmonary emboli. Clot progression as determined by duplex scanning did not predict acute complications of deep venous thrombosis. 