A case is reported of a 14 month old boy with severe dietary vitamin B-12 deficiency caused by his mother's vegan diet. Clinical, electroencephalography (EEG), and haematological findings are described. Cranial magnetic resonance imaging (MRI) showed severe frontal and frontoparietal cranial atrophy. Vitamin B-12 supplements led to a rapid improvement of haematological and neurological symptoms. Serum vitamin B-12 and urinary methylmalonate excretion were normal 10 days after treatment began. After six weeks, EEG was normal and cranial MRI after 10 weeks showed complete disappearance of all structural abnormalities. Cognitive and language development, however, remained seriously retarded at the age of 2 years. It is concluded that infantile vitamin B-12 deficiency induced by maternal vegan diets may cause lasting neurodisability even though vitamin B-12 supplementation leads to rapid resolution of cerebral atrophy and electroencephalographic abnormality.