Physical exercise may deplete magnesium, which together with a marginal dietary magnesium intake may impair energy metabolism, muscle function, oxygen uptake and electrolyte balance. Consequently, the ability to perform physical work may be compromised. Many aspects of immune function can be depressed temporarily by either a single bout of very severe exercise or a longer period of excessive training. Although the disturbance is usually quite transient, it can be sufficient to allow a clinical episode of infection, particularly upper respiratory tract infections. However, regular and moderate exercise has been reported to improve the ability of the immune system to protect the host from infection. Magnesium also has a strong relation with the immune system in both non specific and specific immune responses and magnesium deficit has been shown to be related to impaired cellular and humoral immune function. Magnesium deficiency leads to immunopathological changes that are related to the initiation of a sequential inflammatory response. Although in athletes magnesium deficiency has not been investigated regarding alterations in the immune system, the possibility exists that magnesium deficiency could contribute to the immunological changes observed after strenuous exercise.