Many components of the immune system exhibit adverse change after prolonged, intense exertion. During this "open window" of impaired immunity (which may last 3-72 h, depending on the immune measure), viruses and bacteria may gain a foothold, increasing the risk for subclinical and clinical infection. The influence of nutritional supplements, primarily zinc, vitamin C, glutamine, and carbohydrate, on the acute immune response to prolonged exercise has been measured in endurance athletes. Vitamin C and glutamine have received much attention, but the data thus far are inconclusive. The most impressive results have been reported with carbohydrate supplementation. Carbohydrate beverage ingestion has been associated with increased plasma glucose levels, an attenuated cortisol and growth hormone response, fewer perturbations in blood immune cell counts, decreased granulocyte and monocyte phagocytosis and oxidative burst activity, and a diminished pro-inflammatory and anti-inflammatory cytokine response. Overall these data indicate that the physiologic stress to the immune system is reduced when endurance athletes use carbohydrate beverages before, during, and after prolonged and intense exertion. The clinical significance of these carbohydrate-induced effects on the endocrine and immune systems awaits further research.