One hundred and eleven patients with venous thrombosis were studied with pelvic phlebography and 99mTc-MAA venous scintigram. Based upon the degree of iliac venous compression, they were classified into 4 types: Normal type (I), slightly compressed type (II), moderately compressed type (III) and severely compressed type (IV). Left ilio-femoral venous thrombosis frequently occurred in type III and type IV, which suggests that left ilio-femoral venous thrombosis tends to develop with the advance of impaired venous return. The amount of serotonin, one of the substances released from platelets, was measured after the ligation of iliac veins in 31 mongrel dogs. Thrombus formation was observed 3 days after the ligation, while the decline of intra-platelet serotonin and the rise of extra-platelet serotonin had already been seen 3 hours after the ligation. In all the 18 patients who had laparotomy, intra-platelet serotonin levels declined significantly 3 days after the operation, and left ilio-femoral venous thrombosis with iliac venous compression was detected in three of them without any symptoms. CONCLUSIONS Iliac venous compression tends to produce left ilio-femoral venous thrombosis not only under impaired venous return but also from the releasing reaction of platelets, suggesting a strong possibility that platelets may play an important role in venous thrombogenesis.