The metabolism of acetaldehyde has received considerable attention in the past few years due to its toxic effects and possible importance in pharmacogenetics. Recent studies have demonstrated rapid progress concerning the multiple molecular forms of ADH and ALDH and their genetic variants. The isozymes of ALDH may play an important role in the biological sensitivity to alcohol in certain ethnic groups and also in the pathogenesis of alcohol related organ damage. A protective effect of ALDH I deficiency against alcoholism seems to exist in Japanese.