The dependence of bronchial reactivity to exercise on baseline lung function was studied in 84 asthmatic children aged 7-16 years. The percentage fall in PEF values after a standard exercise running test was 8.4 +/- 8.6% in the 39 children with an attack rate of less than 10 per year and 29.6 +/- 23.2% in the 45 children with an attack rate of 10 or more per year (p less than 0.001). A significant negative correlation (r = -0.46, p less than 0.001) was found for pre-exercise MMEF and other sensitive tests of airway calibre with the response to exercise, but no such correlation was found between baseline PEF or SGaw values and the exercise response. These results show that bronchial hyperreactivity to exercise is dependent on residual airway obstruction, but a wide variety of reactivity can occur. If the baseline flow-values are less than three standard deviations below the mean reference, however, a clinically significant response to exercise can be predicted.