This report describes a case of congenital dyserythropoietic anemia (CDA) type I diagnosed in a 22-year-old woman with a two year history of macrocytic anemia. Light microscopy study of bone marrow disclosed only minor and nonspecific findings, without erythroid hyperplasia. Electron microscopy of bone marrow erythroblasts surprisingly showed most of the classical features of CDA type I. Electron microscopy may be an important tool for the diagnosis of macrocytic anemia of obscure origin, especially in young patients.