Acute exercise of varying severity and corresponding levels of long term competition and training have been found to affect various components of the immune system including lymphocyte subsets, immunoglobulin levels, the mononuclear phagocytic system, polymorphonuclear leukocytes and cytokines, especially IL-1, IL-2, IL-6 and TNF. A tentative trend may be discerned whereby light to moderate exercise may increase immune responsiveness but high-level competition sport, especially if it involves extensive endurance training, may lead to a degree of immunosuppression. Such immune malfunction may be a component of the overtraining syndrome, in which recurrent infections during periods of maximum training or competition stress may form part of the syndrome. Evidence is presented that such overworked muscle may fail to supply adequate glutamine for normal lymphocyte function. Principles of an overtraining treatment strategy are suggested.