Cobalamin C (Cbl-C) defect is the most common inborn error of cobalamin metabolism which causes a block in the pathway responsible for the synthesis of its two metabolically active forms methyl- and adenosylcobalamin. Cbl-C defect causes the accumulation of methylmalonic acid and homocysteine and decreased methionine synthesis. The clinical presentation of patients with early-onset Cbl-C defect, characterized by a multisystem disease with severe neurological, ocular, hematological, renal, gastrointestinal, cardiac, and pulmonary manifestations, differs considerably from what observed in the “classical” form of methylmalonic aciduria caused by defect of methylmalonyl-CoA mutase. This last condition is in most cases dominated in the neonatal period by a metabolic encephalopathy “intoxication type” with severe hyperammonemia and ketoacidosis. We report a Cbl-C defect patient presenting a neonatal encephalopathy with severe hyperammonemia and ketoacidosis who was successfully treated with peritoneal dialysis. Conclusion: To the best of our knowledge, there are no reported cases of Cbl-C defect showing an acute presentation resembling a classical methylmalonic aciduria. This observation enlarges the spectrum of inherited diseases to be considered in the differential diagnosis of neonatal hyperammonemia.