Aldehyde dehydrogenases (ALDHs), a superfamily of NAD(P)+-dependent enzymes with similar primary structures, catalyze the oxidation of a wide spectrum of endogenous and exogenous aliphatic and aromatic aldehydes. Thus far, 16 ALDH genes with distinct chromosomal locations have been identified in the human genome. Polymorphism in ALDH2 is associated with altered acetaldehyde metabolism, decreased risk of alcoholism and increased risk of ethanol-induced cancers. Polymorphisms in ALDH3A2, ALDH4A1, ALDH5A1 and ALDH6A1 are associated with metabolic diseases generally characterized by neurologic complications. Mutations in ALDH3A2 cause loss of enzymatic activity and are the molecular basis of Sjögren-Larsson syndrome. Mutations in ALDH4A1 are associated with type II hyperprolinemia. Deficiency in ALDH5A1 causes 4-hydroxybutyric aciduria. Lack of ALDH6A1 appears to be associated with developmental delay. Allelic variants of the ALDH1A1, ALDH1B1, ALDH3A1 and ALDH9A1 genes have also been observed but not yet characterized. This review describes consequences of ALDH polymorphisms with respect to drug metabolism and disease.