The relationship between exercise and upper respiratory tract infections (URTI) may be modeled in the form of a "J" curve. Various epidemiological studies suggest that unusually heavy acute or chronic exercise is associated with an increased risk of URTI. The risk appears to be especially high during the one or 2-wk period following marathon-type race events. Among runners varying widely in training habits, the risk for URTI is slightly elevated for the highest distance runners, but only when several confounding factors are controlled for. Two randomized experimental trials using small numbers of subjects have provided important preliminary data in support of the viewpoint that moderate physical activity may reduce URTI symptomatology. Clinical data support the concept that heavy exertion increases the athlete's risk of URTI because of negative changes in immune function and elevation of the stress hormones, epinephrine, and cortisol. On the other hand, there is growing evidence that moderate amounts of exercise may decrease one's risk of URTI through favorable changes in immune function without the negative attending effects of the stress hormones.