Classically, deficiency of folic acid (folate) or vitamin B12 (cobalamin) was recognized by the presence of a macrocytic anemia resulting from megaloblastic changes in the bone marrow. A markedly changing paradigm has identified both new mechanisms for altered folate and cobalamin status and new sequelae and clinical interrelationships that include altered mechanisms of absorption, a changing pattern of neurologic deficits, an increased risk of vascular occlusive lesions, and an important relationship with the mechanisms of neoplastic transformation. Several of these newer characterizations relate to issues of neoplasia in the nonpregnant woman and to issues in pregnancy, such as the potential for developmental abnormalities of the fetal nervous system.