A case of hereditary transcobalamin II deficiency with neurological involvement is described. The patient presented in early infancy with megaloblastic anaemia and was treated with folinic acid from 6 weeks of age. The diagnosis of transcobalamin II deficiency was not made until he was 2 years old when he showed severely retarded intellectual development, ataxia and pyramidal deficit in the limbs. Following treatment with hydroxocobalamin, his condition has slowly improved but he has remained with a severe neurological deficit. The consequences of vitamin B12 deficiency on neurological development in infancy are discussed.