We report case of an infant who presented with failure to thrive and developmental delay at 4 months of age. He was diagnosed to have vitamin B12 deficiency and antibodies to intrinsic factor secondary to undiagnosed maternal pernicious anemia. The child was treated with hydroxocobalamin and now at 2 years of age, he is developing and growing within normal range. We review the literature on this rare cause of cobalamin deficiency in infants. We highlight the factors determining the outcome and situations where raised index of suspicion could help in recognizing this preventable cause of developmental delay and learning difficulties.