We studied the protective effect of sustained-release theophylline granules on early or late response in exercise-induced asthma (EIA) in 16 patients ranging in age from 7 to 18 years. A standardized exercise test was examined 2 hours after administration of theophylline granules on one day and after no medication on another day. On both days, the lung functions and the serum theophylline and cortisol concentrations of the patients were examined before exercise and over a period of 8 hours after exercise. On days without medication, early response was seen in 16 patients and late response in 6. On days with medication, the theophylline granules inhibited early response in 13 of the 16 patients and late response in 5 of the 6 patients. This suggests that theophylline preparation is a useful drug for inhibiting the dual responses of EIA.