We report two infants with an inborn error of cobalamin (vitamin B(12)) metabolism whose clinical presentation in the first month of life strongly suggested bacterial or viral sepsis. The absence of any acute metabolic derangement (acidosis, hyperammonemia, hypoglycemia, or ketosis) in association with clinical features suggesting sepsis (lethargy, obtundation) could impede the correct diagnosis of cobalamin C (cblC) disorder. In addition, this is the first documentation of cerebrospinal fluid hyperhomocysteinemia in cblC defect that was highly increased and is likely to be associated with neurotoxicity in cblC patients.