Erythrocyte width is the difference between different red blood cell volumes, for example, the volume between the largest red blood cell and the smallest red blood cell for comparison, the larger the difference, the greater the red blood cell width. The increase in red blood cell width can be classified as follows: 1. The appearance of smaller red blood cells with an increase in their width is mostly seen in iron deficiency anemia. Patients with iron deficiency anemia are microcytic hypochromic anemia, and the red blood cell volume becomes smaller, while the normal cells do not change, so the width increases. 2. The red blood cells become larger, mostly seen in megaloblastic anemia. The patient’s red blood cell volume becomes larger while the normal cells remain unchanged, so the width also increases. 3. Red blood cells become both smaller and larger, and the width increases even more, as in mixed anemia, where iron deficiency is accompanied by folic acid and vitamin B12 deficiency. 4. Patients with some diseases, such as hemolytic anemia and myelodysplastic syndrome, can also experience an increase in red blood cell width due to the disease state. Therefore, the increase in red blood cell width is a patient suffering from a disease, which is a pathological manifestation, not a physiological one. Once a patient has an increased erythrocyte width, it is important to go to a hematology clinic to see a specialist to find the cause and deal with it in a timely manner to avoid delaying treatment and leading to unnecessary consequences. Therefore, an increase in the width of red blood cells can be caused by many diseases and needs to be alerted and not ignored.