A 7-month-old male presented with lethargy and failure to thrive. The child was exclusively breast-fed from birth by a mother who was a strict vegetarian. Laboratory data revealed macrocytic anemia and methylmalonic acid in the urine, consistent with vitamin B12 deficient anemia. The patient responded well to supplementation with B12 alone and was developmentally normal by 11 months of age. This study emphasizes the need for assuring maternal dietary adequacy during pregnancy and after birth.