A review of exercise-induced asthma is presented which describes work that has been carried out by the authors and by other investigators over recent years. The effect of exericse on lung function in asthmatic and normal subjects is compared. The influence of the type and severity of exercise on the response of the asthmatic is noted and the importance this has for interpretation of results. The effects of various drugs on exercise-induced asthma are considered in some detail. The clinical implications of the results of exercise tests in asthmatics, their relatives, and other subjects are considered in terms of the diagnosis and prognosis of asthma and its mode of inheritance. It is concluded that there is as yet no explantation for the mechanism of exercise-induced asthma, but it is a tool of potentially great value for research into the physiology and treatment of clinical asthma.