A high erythrocyte volume means that the volume of red blood cells is higher than the normal range. The normal volume of red blood cells is 80-100 fL, and more than 100 fL is a high erythrocyte volume. The most common cause of high red blood cell volume is a condition called megaloblastic anemia. Due to a deficiency of folic acid and vitamin B12, the nucleus of the red blood cells does not divide properly, resulting in an increase in red blood cell volume. In addition, other diseases, such as hemolytic anemia or myelodysplastic syndrome, may also result in high red blood cell volume. Therefore, increased erythrocyte volume can be seen in a variety of diseases, including megaloblastic anemia, myelodysplastic syndrome, and hemolytic anemia, which are the most common types of diseases. A differential diagnosis is required, and a definitive diagnosis of megaloblastic anemia and myelodysplastic syndrome can be made by bone marrow aspiration. The diagnosis of autoimmune hemolysis can also be made by acid hemolysis test and anti-human globulin test.