The case of a young girl, born to a woman who was a vegetarian for 18 years, is presented. She had been exclusively breast-fed until the age of 6 months when a severe anemia was discovered with an extremely low hemoglobin level (1.9 g/100 ml). Her physical growth and psychomotor development had been normal until 3 months of age. Bone marrow showed megaloblastosis and the serum B12 level was low (45 ng/l). B12 levels were also decreased in both parents (110 and 105 ng/l) and in the mother's milk (12 ng/l). Treatment with parenteral B12 was successful. The importance of a careful dietetic inquiry in the case of an infant with megaloblastic anemia is stressed and likewise, as a preventive measure during all normal pregnancies.