Exercise challenges were performed on 397 middle and high school athletes to detect unrecognized exercise-induced bronchospasm. All challenges were completed by a standard treadmill protocol and spirometry was performed prior to exercise and at 1, 10, 20, and 30 minutes after completion of the treadmill regimen. Using FEV1 as the diagnostic standard, we compared the number of students who had initial decreases of > or = 10%, > or = 15%, or > or = 20% at each of the four postexercise spirometric evaluations. FEV1 thresholds of > or = 10%, > or = 15%, and > or = 20%, when analyzed, identified 187, 125, and 90 subjects respectively with a positive response. The majority of those with a positive response were identified soon after completion of the exercise provocation; however, a larger than expected number of athletes had their initial drop in FEV1 > or = 20 minutes after cessation of exercise. This represented 9% to 14% of the total number of responders to exercise challenge. Protocols for evaluation of exercise-induced bronchospasm may need to be designed to include data points up to or beyond 30 minutes after exercise to avoid missing the late appearance of bronchospasm.