Aim Nutritional B12 deficiency is a treatable cause of neurodevelopmental delay in infants. We report 21 infants with developmental regression and brain atrophy as revealed using cranial magnetic resonance imaging (MRI), secondary to severe vitamin B12 deficiency. Methods Twenty-one infants aged 4-24 months with B12 deficiencies who were admitted to our clinic between May 2013 and May 2018 were included in the study. MRI, bone marrow aspiration and the Denver-II Developmental Screening Test were performed in all infants. Results The mean age of the infants was 12.3 months, and the mean B12 level was 70.15 +/- 32.15 ng/L. Hypotonia and neurodevelopmental retardation, and anaemia were present in all patients. Their bone marrow examinations were compatible with megaloblastic anaemia. Twelve patients had microcephaly, seven had tremor and one patient died of severe sepsis. Almost all patients were fed with breast milk and their mothers were also malnourished. Nine (42.9%) of the patients were Turkish and 12 (57.1%) were Syrian. All patients had abnormal Denver-II Developmental Screening Test scores. Most patients had severe cortical atrophy, cerebral effusion, thinning of the corpus callosum and delayed myelinisation in cranial MRI. Treatment with B12 resulted in dramatic improvement in general activity and appetite within 72 h. Tremors resolved in all cases. Conclusion Neurological findings and developmental delay related to nutritional B12 deficiency can be prevented without sequelae if diagnosed early. Screening and treating of mothers for this deficiency will contribute to the health of both the mother and their feeding infant.