Zinc is essential for many metabolic and enzymatic functions in man. Deficiency of zinc in man has now been recognized to occur not only as a result of nutritional factors, but also in various disease states, including malabsorption syndromes, acrodermatitis enteropathica, Crohn's disease, alcoholism and cirrhosis of the liver. The deficiency state in human subjects exists as a spectrum extending from mild to severe degree. The clinical manifestations of mild zinc deficiency include oligospermia, weight loss and hyperammonaemia. Moderate zinc deficiency is characterized clinically by growth retardation, hypogonadism in males, skin changes, poor appetite, mental lethargy, delayed wound healing, taste abnormalities and abnormal dark adaptation. In severe zinc deficiency states, bullous-pustular dermatitis, alopecia, diarrhoea, emotional disorders, weight loss, intercurrent infections, hypogonadism in males and, if unrecognized, death have been observed. Zinc is needed for the functions of over 100 enzymes. It is essential for DNA, RNA and protein synthesis and, as such, is important for cell division. Zinc is an inducer of mRNA of metallothionein, a protein which may have an important role in the regulation of intestinal zinc absorption. Zinc has a specific effect on testes in animals and man. Recent reports indicate that in human subjects thymopoietin may be zinc dependent and in animal studies somatomedin may be affected adversely due to dietary zinc restriction. Zinc plays an important role in the protection of cell membrane integrity and may be protective against free radical injury. Zinc is known to compete with cadmium, lead, copper, iron and calcium for similar binding sites. In the future, a potential use of zinc may be to alleviate toxic effects of cadmium and lead in human subjects. Recent evidence suggests that thymic-dependent lymphocytes (T cells are zinc dependent. T-helper and suppressor cells, T-effector cells and T-natural killer cells appear to be zinc dependent. Zinc is also essential for some of the neutrophil functions. Thus, it appears that zinc may play an important role in immunity. One may suggest that some of the clinical features of cirrhosis of the liver, such as testicular atrophy, loss of body hair, night blindness, poor wound healing, poor appetite, susceptibility to infections and enhanced sensitivity to drugs, may be related to conditioned deficiency of zinc, future studies are required to determine whether or not zinc supplementation is beneficial to these patients.