In addition to degree of outflow obstruction and reflux and poor calf muscle pump function, vein wall compliance changes are important in understanding pathophysiology of venous disease. This study compares the pressure/volume relationship in post-thrombotic and healthy legs.Prospective and comparative study.Vascular laboratory.Investigations were performed on 24 apparently normal legs and 30 post-thrombotic limbs as confirmed by phlebography.Dorsal vein pressure and absolute calf volume decrease were recorded simultaneously during outflow form the leg after release of the cuff during venous occlusion air-plethysmography with and without reactive hyperemia. The slope of the pressure/volume outflow curve was calculated. In addition, the distensibility (= collapsibility) was determined as % volume decrease/mmHg. The popliteal and femoral vein diameters were measured in supine and erect position by ultrasound.With and without induced hyperemia the mean slope coefficients of post-thrombotic legs were significantly higher (0.52 +/- 0.22 and 0.53 +/- 0.18) than in normal (0.15 +/- 0.10 and 0.29 +/- 0.11), i.e., the curve steeper since the calf was stiffer, less compliant. The degree of outflow obstruction and severity of skin changes did not affect the slope measurement substantially. Collapsibility during venous outflow was significantly less in post-thrombotic legs. The post-thrombotic veins were less distended on standing.Post-thrombotic calves are less compliant than normal legs mainly due to less compliance of the vein wall, although theoretically reduced venous volume may contribute.