Hydroxyacyl-Coenzyme A dehydrogenase also known as HADH is an enzyme which in humans is encoded by the HADH gene. This gene is a member of the 3-hydroxyacyl-CoA dehydrogenase gene family. The encoded protein functions in the mitochondrial matrix to catalyze the oxidation of straight-chain 3-hydroxyacyl-CoAs as part of the beta-oxidation pathway. Its enzymatic activity is highest with medium-chain-length fatty acids. Mutations in this gene cause one form of familial hyperinsulinemic hypoglycemia. A deficiency is associated with 3-hydroxyacyl-coenzyme A dehydrogenase deficiency.