The authors describe a 16-month-old infant presenting with neurologic developmental regression, severe pancytopenia, excessive skin pigmentation, and tremor resulting from nutritional vitamin B12 deficiency. She had been exclusively breast-fed and had refused to take any other food. Laboratory studies showed severe pancytopenia, a decrease in serum B12 levels, and an increase in urinary methylmalonic acid levels. Bone marrow aspiration was compatible with megaloblastic changes. Schilling test was normal. The serum B12 level of the mother was also low. Megaloblastic anemia resulting from inadequate B12 intake was diagnosed. Parenteral B12 therapy was initiated. The neurologic picture did not completely resolve, but pancytopenia, tremor, and hyperpigmentation of the extremities recovered completely.