Aldehyde dehydrogenase (ALDH) consists of four different isozymes (I, II, III, IV). Among these, ALDH I shows genetic polymorphism (normal and deficient) in a population of Mongoloid origin. The significantly lower frequency of ALDH I deficiency was found in alcoholic patients compared with healthy controls. ALDH I deficiency is one of the important genetic factors in regulating alcohol consumption and plays a protective role against alcoholism. Population genetic studies on ALDH I deficiency in different ethnic groups indicate that ALDH deficiency was found only in Mongoloid population groups.