Combined malonic and methylmalonic aciduria (CMAMMA) is an inborn error of metabolism characterized by elevations of urinary malonic acid (MA) and methylmalonic acid (MMA). In the classic form, MA elevations are typically higher than MMA, and affected individuals have normal malonyl-CoA decarboxylase enzyme activity. CMAMMA has a broad phenotypic spectrum, presenting in childhood and in adults. Most affected individuals have metabolic acidosis, developmental delay, seizures, and cardiomyopathy. Other findings reported include: coma, hypoglycemia, failure to thrive, immunodeficiency, microcephaly, dystonia reported in children, whereas adults have also presented with psychiatric disease, memory problems, and cognitive decline. The incidence of CMAMMA has been estimated at approximately 1 in 30,000. However, some affected individuals exhibit no clinical symptoms and the disorder is believed to be under-diagnosed.