In this review we discuss four genetic disorders of copper metabolism. Wilson's disease and Indian childhood cirrhosis result from the toxic effects of copper accumulation in the liver. Menkes' disease and, most likely, occipital horn syndrome result from copper deficiency secondary to disturbances in copper transport. The recent cloning and sequencing of the genes defective in Wilson's disease and Menkes' disease provide the molecular basis for understanding the causes of the two major disorders of copper transport in humans. Mutations that result in Wilson's and Menkes' diseases were shown to disrupt the function of two related P-type copper transporting ATPases. Genetic analysis demonstrates that Wilson's disease and, probably, Menkes' disease are caused by a number of different mutations within a single gene (allelic heterogeneity), and that this occurrence likely explains the clinical heterogeneity of both diseases. The possibility that different mutations within the same gene account for the similar phenotypes of Wilson's disease and Indian childhood cirrhosis on the one hand and for Menkes' disease and occipital horn syndrome on the other are discussed.