Vitamin B12 deficiency usually occurs in exclusively breastfed infants whose mothers have pernicious anaemia or are vegetarian. Early treatment of vitamin B12 deficiency in infants can prevent potentially neurologic sequelae. A male child aged 13 months has been hospitalized due to failure to thrive, feeding problems, pallor, weakness and hypotonia. During the pregnancy mother did not eat meat and during lactation she also excluded eggs and milk. The child was exclusively breastfed. Laboratory investigations showed a haemoglobin level of 3.5 g/dL, haematocrit 10%, red blood cell count of 0.99 × 10 12 /L, white blood cell count of 4.23 × 10 9 /L and platelet count of 55 × 10 9 /L. Vitamin B12 level was low. A bone marrow aspiration finding was consistent with megaloblastic anaemia. The magnetic resonance imaging showed brain atrophy. Vitamin B12 in a dose of 10µg per kg was applied intramuscularly daily for 2 weeks, then once weekly. Three days after initiating B12 vitamin therapy there was an improvement in the blood count with the gradual improvement of neurological state. Vitamin B12 deficiency is a treatable cause of pancytopenia and neurological dysfunction in children and should be considered as differential diagnosis in an infant with neurological symptoms.