Thrombophlebitis may occur as a primary disease in healthy, active persons who otherwise are normal. In a series of one thousand and eleven unselected consecutive patients with thrombophlebitis of all types seen at the Mayo Clinic in a five year period, seventy-nine formed the basis for the study under discussion. The patients had no evidence of constitutional disease, such as carcinoma or arthritis. They had not undergone surgical operation or childbirth recently and had not recently had a specific infectious disease. They did not have recognizable disease of the heart or dyscrasia of the blood. There was no history or evidence of gross mechanical trauma or of local infectious or suppurative processes in the region of the veins. The patients had not had previous disease of the veins, such as varix. They were examined carefully for occlusive disease of the peripheral arteries, but this was not found. In other words,