The vitamin B12 status of infants depends on maternal B12 status during pregnancy, and during lactation if breastfed. We present a 9-month-old girl who was admitted to the metabolic unit for assessment of developmental delay. She was exclusively breastfed and the introduction of solids at 5 months was unsuccessful. Investigations revealed pancytopenia, undetectable B12 and highly elevated methylmalonic acid and homocysteine. Methylmalonic acid and homocysteine normalised following B12 injections. Marked catch-up of developmental milestones was noted after treatment with B12. Investigations of parents showed normal B12 in the father and combined B12 and iron deficiency in the mother. Maternal B12 deficiency, most likely masked by iron deficiency, led to severe B12 deficiency in the infant. Exclusive breastfeeding and a subsequent failure to wean exacerbated the infant's B12 deficiency leading to developmental delay. This case highlights the need for development of guidelines for better assessment of B12 status during pregnancy.