A 4-year-old Japanese girl had hemolytic anemia, thrombocytopenia, distorted and fragmented red cells in the peripheral blood, and megakaryocytosis in the bone marrow from the newborn period. These features were temporarily relieved and prevented by the infusion of fresh frozen plasma from healthy donors. This would indicate a deficiency in the patient's plasma of factor(s) that inhibit platelet aggregation in the blood vessels. An attempt to demonstrate possible in vitro deficiency of the platelet aggregation-prohibiting factor in the patient's plasma was unsuccessful. Laboratory examinations ruled out red cell fragility, abnormalities of red cell enzymes, abnormal hemoglobins, or immune reaction as underlying mechanisms. Our patient, together with three other patients in the literature with similar clinical features, may constitute a clinical entity that could be called "congenital microangiopathic hemolytic anemia".