Alcohol is metabolized by two pathways in humans: the ADH pathway which accounts for the bulk of the metabolism, and the MEOS pathway which contributes to the increased rate of ethanol elimination at high blood alcohol levels. The increased rate of elimination which results from chronic alcohol consumption is due to an increase in MEOS activity. The activities of these pathways are influenced by environmental factors such as smoking, diet, and endocrine factors. In addition, individuals inherit different types of ADH isoenzymes which have different kinetic properties. Individuals with different phenotypic variants, e.g. the beta 1 vs beta 2 isoenzymes, appear to have different rates of ethanol elimination. The cloning of the ADH genes and the availability of molecular hybridization methods now make it possible to genotype individuals and to correlate the genotype with both alcohol elimination rates and with the risk of developing medical complications of alcoholism or even of developing alcoholism itself.