In 33 patients with acute deep venous thrombosis of the lower extremity, all verified by phlebography, and in 36 healthy subjects arterial inflow, venous volume and maximum venous outflow were measured by mercury-strain gauge plethysmography. The measurements were performed simultaneously in both legs at the calf and foot level with a cuff pressure of 60 mm Hg for 4 min. In contrast to thrombotic occlusions proximal to the knee and multi-level thromboses, which could be identified by a significantly (p less than 0.001) reduced venous volume and maximum venous outflow (measurement at the calf level), isolated calf vein thromboses could not be detected even by sensing from the foot level or only if all three deep veins of the calf were occluded. The best diagnostic criterion for proximal deep venous thrombosis was the correlation of maximum venous outflow and venous volume (83% right positive), if these parameters were determined from the calf. The results indicate that deep calf vein thrombosis can be detected, even if sensed from the foot, only in cases with cross sectional thrombotic occlusions.