Methylmalonic aciduria stands for increased excretion of methylmalonic acid in urine for which the underlying cause can be a group of rare, autosomal recessive disorders of branched-chain amino acid metabolism. It is one of the most common organic acidurias. The defect can be due to the lack of the enzyme methylmalonyl-CoA mutase activity or to defects in cobalamin metabolism. Consequently, there is an accumulation of methylmalonic acid and it's precursors which are responsible for the symptoms of the disease. Clinical presentation is nonspecific, symptoms can occur from neonatal period and infancy up to adulthood, but usually they present in the first year of life in the form of acute metabolic crisis. If the acute crisis is untreated it may progress to coma or death. Long-term complications include developmental delay, intellectual disability, renal complications and various neurological and hematological complications. The diagnostic approach should include a thorough analysis of the clinical presentation and course of the disease in the first place, the analysis of organic acids in the urine, and then the determination of the specific subtype of methylmalonic aciduria by enzymatic analysis, followed by testing cobalamin responsivness and gene analysis. In countries with an expanded newborn screening program it can also be an important element in diagnosis. Long-term management is aimed at preventing metabolic decompensation. Patients are placed on a protein-restricted diet, given carnitine supplements and cobalamin if cobalamin responsive. More recent, liver, or combined kidney-liver transplantation is being talked about as a way of treatment of the most severe cases of methylmalonic aciduria.