The decrease in pulmonary function, expressed as peak expiratory flow, which is seen in some asthmatics after exercise (exercise-induced asthma (EIA)), has been studied under conditions standardized with regard to ventilation during exercise, room temperature and relative humidity of the inhaled air. Exercise was performed under four different conditions. 1) temperature (T) 15 degrees C and relative humidity (RH) 30%, and 2) T 15 degrees C and RH 70%, 3) T 30 degrees C and RH 30% and 4) T 30 degrees C and RH 70%. Treadmill running was performed on four succeeding days at the same time of day with the four different combined conditions. Identical ventilation during the exercise was secured on each day by monitoring respiratory frequency, tidal volume, minute ventilation and accumulated ventilation. A significantly smaller decrease in pulmonary function occurred when both T and RH were high, whereas the decrease in pulmonary function after exercise was identical when either temperature or relative humidity, or both, were low. It is concluded that the water concentration of the inspired air is negatively correlated to the decrease in pulmonary function after exercise in asthmatics.