A 6-week period of physical training of 28 asthmatic children reduced the exercise-induced percentage fall in peak expiratory flow (from baseline) from 44 +/- 4% (mean +/- SE) to 30 +/- 4%. Resting pulmonary function was unchanged. Resting and maximum heart rate (submaximal treadmill running) and postexercise plasma lactate were all lowered by training. The individual values for exercise-induced bronchoconstriction and postexercise plasma lactate correlated positively before and after training. Furthermore, the training-induced reduction in these parameters correlated. In 14 asthmatic children, who served as controls, exercise-induced bronchoconstriction and physical fitness remained unchanged during a comparable period. The study demonstrates a beneficial effect of endurance training on exercise-induced bronchoconstriction and working capacity.