Introduction: Innate deficits in the metabolism or transport of vitamin B12 are exceptional. The main cause of vitamin B12 deficiency in infants is secondary to maternal deficiency. Maternal deficiency can have a vegan diet, low socio-economic level (developing countries) and a high level of development with a poor diet) or digestive pathologies responsible for a lack of absorption. The clinical signs are not very specific (developmental delay, pallor, hypotonia, vomiting and diarrhoea), which explains the frequent delay in diagnosis. The pathophysiology of the neurological damage, which is still uncertain, is thought to be secondary to a defect in myelination with altered nerve conduction and cortical atrophy. The aim of this work is to underline the importance of raising awareness of vitamin B12 deficiency in order to avoid its profound neurological repercussions, especially as substitution treatment allows an improvement or a reversibility of the neurological damage and raises the question of the possibility of screening new-borns. Materials and Methods: We report the case of two infants aged 6 months and 10 months with hypotonia and psychomotor regression from an early age, whose biological examination revealed pancytopenia with a collapsed vitamin B12 level in both infants and both mothers.