It is usual to associate megaloblastic anemia with folate or cobalamin deficien- cies. However, this notion, even if true in most cases, is too restricted. Megalo- blastosis in blood may also be observed in blood diseases without vitamin defi- ciency, and also after treatment with certain antineoplastic agents; in these con- ditions, the mechanisms vary with the etiology. On the other hand, folate or cobalamin deficiency may induce various clinical or biochemical disturbances without - as yet - macrocytic megaloblastic anemia. That the biochemical basis of megaloblastosis is the same in folate and cobal- amin deficiencies is due to the close metabolic interrelationships between thse two vitamins. However, the role of cobalamin deficiency in folate metabolism is still a matter of debate. Morphological abnormalities such as macrocytosis in peripheral blood and megablastosis in bone marrow, long considered to be the best indices of vitamin deficiency, are not always constant. Indeed, the improved diagnostic methods often lead to an early diagnosis of deficiency before the appearance of the usual hematological abnormalities.