Twenty-five normal and 105 asthmatic children were exercised on a treadmill. Pulmonary function was assessed before and after exercise. The maximum fall from the resting value in normal subjects depended on the test used: PEFR 12.5%; FEV1 10%; MMEF 26%; V50 30%; V25 33%. Using these criteria, PEFR and FEV1 detected 99% of those asthmatic children who had a positive exercise response. The largest fall from the resting value was seen with the MMEF, but this test detected only 70% of the positive responders. The pre-exercise function did not affect the severity of the response but did have an effect on the incidence of exercise-induced bronchospasm.