The case of a 15-month-old, strictly breast-fed infant whose mother had been following a vegetarian diet for ten years is reported. The infant had severe megaloblastic anemia with an arrest in growth, hypotonia, and failure of psychomotor development. The very low levels of vitamin B12 in the infant's serum and mother's milk confirmed the diagnosis. Management of such cases consists in administration of vitamin B12 supplements, with a blood transfusion if needed. Other concomitant deficiencies should be looked for. The outcome is rapidly favorable. The patient reported here is now four years of age and has normal statural growth and psychomotor development.