Opinions differ as to whether marathon runners have an increased susceptibility to upper respiratory tract (URT) infections after a race. In an attempt to answer this question, we carried out a prospective study of the incidence of symptoms of URT infections in 150 randomly selected runners who took part in the 1982 Two Oceans Marathon in Cape Town, and compared this with the incidence in individually matched controls who did not run. Runners were questioned on the day before and 2 weeks after the race. Symptoms of URT infection occurred in 33.3% of runners compared with 15.3% of controls, and were most common in those who achieved the faster race times. The incidence in slow runners was no greater than that in controls. Faster runners also experienced more musculoskeletal pain during and after the race. These results suggest a relationship between acute stress and susceptibility to URT infections. Impairment of one or more local mucosal or general host defences may account for this effect.