Introduction: Introduction: the vitamin cobalamin is essential for a proper functioning of the CNS, hematopoiesis, and DNA synthesis. Its deficiency is frequently secondary to pernicious anemia or strict vegetarian diets. Case report: an 18-month-old male infant presented with a decreased level of consciousness and movement disorder (tremor and choreiform movements) of several hours' standing. He had a history of delayed acquisition of motor milestones (not standing, monosyllabic language), and progressive loss of these over the last few weeks (head support and sitting). He had been breastfed from birth. His family has a vegetarian diet. In neuroimages, cranial CT and MRI scans showed generalized supratentorial atrophy involving both matters and the basal ganglia. Treatment was started with intramuscular vitamin B12, which increased its levels. Subsequently, the patient was subjected to dietary diversification and oral cyanocobalamin, with clinical normalization after 6 months and radiological normalization after 7 months. Conclusions: we emphasize the importance of vitamin B12 supplementation during pregnancy and lactation in vegetarina mothers and their infants. [ABSTRACT FROM AUTHOR]