The newborn's vitamin B12 storage exclusively comes from placenta transfer, later from animal food. We relate 3 observations of infants (3-11-13 months) with failure to thrive, anorexia, vomiting and for the two olders refusal of weaning, associated with psychomotricity regression and hypotony. Blood cell count showed a macrocytosis without anemia (case 2-3) and a severe microcytic anemia for the first case caused by a mild alpha-thalassemia, with megaloblastic bone marrow. Vitamin B12 levels were very low associated with increased methylmalonic acid and homocysteine serum levels which confirm the diagnostic . Cerebral imaging showed diffuse cortical atrophy. Cobalamin deficiency was caused by strict vegetarian diets mothers of breastfed infants (cases 2-3) and for younger by mother's unrecognized pernicious anemia. 3 mothers had no anemia and normal B12 's levels at diagnosis. Vitamin B12 supply lead to a rapid clinical and hematologic improvement. In two cases, neurologic recovery was incomplete. About one hundred case of B12 deficiency 's infant are reported, 2/3 are breast-fed by vegetarian mothers, and 1/4 have mothers with pernicious anemia. The failure to thrive is due to anorexia, refusal of weaning and partial villous atrophy. Neurologic manifestations are secondary to cerebral disorders, sometimes revealed by an exposure to anesthetic nitrous oxyd. The macrocytic anemia is inconstant. The etiologic research of developmental delay in an infant may include vitamin B12's deficiency, even if there is no haematologic signs, especially if breast-fedding 's mothers is vegetarian.