ANEMIAS characterized by large erythrocytes (MCV > 95) are usually caused by lack of one of the erythrocyte maturation factors, vitamin B12 or folinic acid. Under these circumstances the young erythrocytes in the bone marrow are megaloblastic and maturation of myeloid elements and megakaryocytes is bizarre. This discussion will be concerned primarily with these anemias (MCV up to 150). There are, however, other causes of macrocytic anemia which deserve mention. These are chronic liver disease, hypothyroidism, and hemolytic and aregenerative anemias. Under these conditions, the size of the erythrocyte is increased only moderately (MCV of 95 to 110) and the bone marrow is normoblastic rather than megaloblastic. These macrocytic anemias are in no way related to the erythrocyte maturation factor deficiency anemias, either etiologically or therapeutically, though the occurrence of large erythrocytes justifies including them in the classification of macrocytic anemias. They will be discussed as a separate group.