Isolated methylmalonic acidemia (AMR) is an inborn error of metabolism due to an enzymatic deficit in methylmalonyl-CoA mutase. AMR lead to increased methylmalonic acid in plasma and urine without hyperhomocysteinemia. The clinical signs are recurrent episodes of ketoacidosis and bouts of vomiting, dehydration and mental retardation. These symptoms do not respond to the administration of vitamin B12. We report a case of a ten-months-old infant to whom the diagnosis was suspected in the presence of a metabolic acidosis, hyperammonemia, without hepatic impairment and ketosis. The chromatography of organic acids showed elevated methylmalonic acid levels. Molecular genetics allowed confirming the diagnosis of deficit in methylmalonyl-CoA mutase demonstrating the genetic abnormality of the gene MUT.