2-Hydroxy-3-methylbutanoic acid (3-Methyl-2-hydroxybutyric acid, 2-Hydroxyisopentanoic acid, 2-Oxyisovaleric acid) is a metabolite found in the urine of patients with phenylketonuria, methylmalonic acidemia, propionic acidemia, 3-ketothiolase deficiency, isovaleric acidemia, 3-methylcrotonylglycemia, 3-hydroxy-3-methylglutaric acidemia, multiple carboxylase deficiency, glutaric aciduria, ornithine transcarbamylase deficiency, glyceroluria, tyrosinemia type I, galactosemia, and maple syrup urine disease.