Five autopsy cases of thrombotic microangiopathy, including 3 cases associated with acute promyelocytic leukaemia, were examined macroscopically, light‐and electronmicroscopically.The so‐called hyaline thrombi In thrombotic microangiopathy were composed of fibrin and its degenerative products. Thrombocytes and other blood cells were not seen in the thrombi.At the site of the formation of a thrombus, there was no conspicuous change in the walls of the capillaries and arterioles. It was considered, therefore, that the intravascular deposition of fibrin was the primary event in the development of thrombotic microangiopathy.In regard to the distribution and morphologic findings, there was no basic difference between the microthrombi in cases associated with acute promyelocytic leukaemia and those without it.The bone marrow and some other organs in cases of thrombotic microangiopathy associated with acute promyelocytic leukaemia macroscopically revealed a green colour. Many thrombi composed of leukaemic cells and fibrin were found in the pulmonary arteries of these cases. Furthermore, prominent erythrophagocytosis in the bone marrow and lymph nodes was a common finding in these ca