Venous occlusion plethysmography (VOP) was originally developed to study the physiology of the arterial circulation in the extremities. Improvements in plethysmographic instruments have now made bedside evaluation of peripheral hemodynamics feasible. In recent years, VOP has been employed for the detection of deep vein thrombosis. Combined analysis of the venous volume increase and the subsequent venous outflow in 3 sec, and measured by impedance plethysmography, has produced a 95% correlation with venography in detecting thrombosis of the popliteal, femoral, and iliac veins in 390 limbs. The method is inadequate for detection of isolated calf thrombi.