Cobalamin and the metabolic markers methylmalonic acid and total homocysteine undergo marked changes during childhood. In breastfed infants a metabolic profile indicative of cobalamin deficiency is common. Symptoms of cobalamin deficiency in children differ with age, presenting a continuum from subtle developmental delay to life-threatening clinical conditions. The symptoms may be difficult to detect, particularly in infants, and there tends to be a diagnostic delay of several months in this age group. Several reports show that even moderate deficiency in children may be harmful, and long-term consequences of neurological deterioration may persist after cobalamin deficiency has been treated. Given the crucial role of cobalamin for normal growth and development, possible widespread infantile deficiency needs attention. Cobalamin deficiency should be considered a differential diagnosis in children with subtle symptoms, and strategies to prevent cobalamin deficiency in mothers and children should be addressed.