Background: Vitamin B12 is essential for nerve and blood cell proper functioning and helps make DNA, the genetic material of the cell. In Indian infants, its deficiency is prevalent but there is limited data available on it. Its early recognition and prompt treatment is necessary to avoid permanent neurological damage. Objectives: To study the relationship between Vitamin B12 deficiency in breast-fed infants and their mothers and its clinico-hematological manifestations. Methods: A case control study was conducted in Department of Pediatrics, Career institute of medical sciences, Lucknow from March 2023 to August 2023 including children between 1-12 months of age and consuming breast milk in their diet. After prior informed consent, all the eligible infants who were hospitalized in the Department of Pediatrics were enrolled for the study. A detailed history and examination was carried out. Malnutrition if present was classified as per WHO classification using the Z scores. Complete blood count and Vitamin B12 level of all enrolled infants and their mothers was done. Vitamin B12 Deficiency would be Defined as: Plasma Vitamin B12 level <200pg/ml (150pmol/L). Results: Among total of 319 infants, 230 infants were anemic and out of them 62 infants (27%) were Vitamin B12 deficient, the relationship between anemia in infant and their vitamin B12 level was statistically significant (p=0.001).Overall the prevalence of vitamin B12 deficincy in study population was found to be 21.3%, the difference between the prevalence of wasting and stunting in Vitamin B12 deficient infants vs non deficient infants was statistically not significant. Of 319 mothers, 72(22.6%) were Vitamin B12 deficient and among them 58 (80.6%) had infants who were also deficient (p<0.001). The odd's of having Vitamin B12 deficient infant if the mother is Vitamin B12 deficient is 98.19 (95% CI 41.5-232.3)