To determine whether asthma is associated with changes in the maximal performance of skeletal muscles, 20 asthmatic and 20 control subjects performed series of 12 maximal static contractions of 15 s duration separated by rest intervals of 15 s (duty cycle, 50%) with the inspiratory and expiratory muscles and a control limb muscle. Muscle strength of the asthmatics did not differ significantly from that of the control subjects. During the series of contractions the maximal and average sustained inspiratory pressures did not decline in control or asthmatic subjects, whereas for both groups of subjects, the expiratory pressures declined about 15%, and elbow force declined about 30%. However, endurance of the expiratory muscles was significantly enhanced in the male asthmatic subjects, who were more severely obstructed than the female subjects. When tested with a duty cycle of 67%, the inspiratory muscles of asthmatic subjects fatigued significantly less than those of control subjects.