In order to define the role of individualized training intensity in a conditioning program for asthmatic children, we have trained seven asthmatics (age = 11.4 +/- 1.8 years) at their ventilatory threshold (VTh) intensity level for a three-month period (aerobic training) and at maximal intensity also for three months (high intensity training). VTh is the point at which a nonlinear increase of VE occurs. Another group of seven asthmatics (age = 11.4 +/- 1.5) served as control subjects. Cardiopulmonary fitness was determined on a cycle ergometer before and after each training session. This study demonstrated that aerobic training, correctly adapted to the child's physical ability, induces the following: (1) a rapid and marked cardiovascular fitness increase; and (2) a decrease in VE over a given work range so that VTh is increased. This is of great importance because hyperventilation is a major determinant of exercise-induced bronchospasm. In contrast, even if high intensity training is well tolerated in an indoor swimming pool, the long-term effects are unsuitable for asthmatic children because the decrease of VTh will involve an increase of hyperventilation, even when exercise is performed at submaximal intensity.