-A case of essential thrombophilia, characterized by intravascular thrombosis without any previously known disease or trauma of the vessels involved, is reported. The pathological changes in the vessels were minimal and of a non-reacting type. The elevation in the platelet count, increase in the globulin fraction of the serum proteins and increased coagulability of the blood plasma were considered as important factors. There was no necropsy evidence of acute infection, and no evidence of heart disease to account for the widespread thrombosis involving the vessels of the right lower extremity, spleen and pancreas. Microscopic examination revealed minimal tissue reaction. In 1937 Nygaard and Brown defined this syndrome and reported 5 cases in which thrombosis occurred in the vessels of the extremities and in the brain, kidney, and heart.