The utilization of venous occlusion plethysmography in the routine diagnosis of acute venous thrombosis in the leg has disclosed a previously unreported type of biphasic venous emptying curve that has proved to be of special diagnostic significance. The examinations were undertaken with a Dohn plethysmograph for segmental application of the calf. The emptying curves in question had an initial phase at the ordinary rapid rate, changing distinctly within 1.5 sec into a definitely slower phase of venous emptying. Biphasic emptying curves were found to be characteristic of patients with isolated, high venous obstruction, mostly acute thromboses, but more or less intact veins in peripheral parts of the leg. The same type of pathological emptying curves could be provoked by external compression of the femoral vein or Valsalva's manoeuvre, providing experimental evidence of the underlying mechanism. The venous emptying course might be similarly influenced also by varicose veins, pregnancy, and right heart failure. The diagnostic experience of biphasic emptying curves in clinical practice indicates, that this particular way of interpreting venous occlusion plethysmography is a valuable complement to phlebography in the detection of proximal venous obstructions in the lower limb, whether due to iliac thrombosis or elusive intrapelvic tumours.