Methylmalonic acidemias, also called methylmalonic acidurias, are a group of inherited metabolic disorders, that prevent the body from properly breaking down proteins and fats. This leads to a buildup of a toxic level of methylmalonic acid in body liquids and tissues. Due to the disturbed branched-chain amino acids (BCAA) metabolism, they are among the classical organic acidemias.

Methylmalonic acidemias have varying diagnoses, treatment requirements, and prognoses, which are determined by the specific genetic mutation causing the inherited form of the disorder.

The first symptoms may begin as early as the first day of life or as late as adulthood. Symptoms can range from mild to life-threatening. Some forms can result in death if undiagnosed or left untreated.

Methylmalonic acidemias are found with an equal frequency across ethnic boundaries.

Depending on the affected gene(s) and mutation, the present symptoms can range from mild to life-threatening.

As a rule, methylmalonic acidemias are not apparent at birth as symptoms do not present themselves until proteins are added to the infant's diet. Because of this, symptoms typically manifest anytime within the first year of life. However, there are also forms that only develop symptoms in adulthood.

Methylmalonic acidemias have an autosomal recessive inheritance pattern, which means the defective gene is located on an autosome, and two copies of the gene—one from each parent—must be inherited to be affected by the disorder. The parents of a child with an autosomal recessive disorder are carriers of one copy of the defective gene, but are usually not affected by the disorder.^{[citation needed]} The exception is methylmalonic acidemia and homocystinuria, cblX type due to variants in HCFC1 gene, which is inherited in an X-linked recessive manner.

The following are the known genotypes responsible for isolated methylmalonic acidemias: