Ventilatory muscles can become fatigued, and this can contribute to respiratory failure. Patients with chronic obstructive lung disease may benefit from improving their ventilatory muscle endurance to improve resistance to fatigue. Ventilatory muscle endurance was measured in 30 normal subjects and 55 patients with cystic fibrosis by finding the highest level of normocapnic hyperpnea that could be sustained for 15 min. Subjects with cystic fibrosis had 36 per cent higher ventilatory muscle endurance than normal subjects, reflecting the chronic training stress of breathing against increased respiratory loads. Four normal subjects and 4 subjects with cystic fibrosis participated in a specific ventilatory muscle endurance training program consisting of 25 min per day of maximal normocapnic hyperpnea 5 days per week for 4 weeks. The cystic fibrosis patients who trained improved their ventilatory muscle endurance by 51.6 per cent, whereas the normal subjects who trained showed a 22.1 per cent increase in ventilatory muscle endurance. Seven subjects with cystic fibrosis participated in a 4-week physical activity training program consisting of at least 1.5 hours per day of intensive swimming and canoeing at summer camp. They increased their ventilatory muscle endurance by 56.7 per cent. There were no other pulmonary function changes. Ventilatory muscle endurance can be readily improved in cystic fibrosis equally well by specific ventilatory muscle endurance exercise.