Familial macrothrombocytopenia with unusually elongated mitochondria

We report a familial case of macrothrombocytopenia without inclusion bodies in polymorphonuclear cells or any congenital abnormalities. The results of the hemostatic and platelet function tests were all normal except for the platelet retention rate. The number of megakaryocytes increased slightly and some were relatively small. Electron microscopic studies revealed a unique morphological abnormality of the platelets' mitochondria.