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...