Respiratory insufficiency is a common cause of morbidity and mortality in patients with Duchenne muscular dystrophy(DMD). In these patients, progressive muscle weakness is a major factor in the development of respiratory insufficiency. Therefore, the physical training program to improve the strength and endurance of respiratory muscle could conceivably improve respiratory function and prevent respiratory complication in patients with DMD. The purpose of this study is to examine the effects of inspiratory muscle training on respiratory function of DMD patients according to functional state. Eighteen DMD patients who were registered at the Muscle clinic of Yong Dong Severance Hospital were assessed for the pulmonary function using the routine pulmonary function test and measurements of maximal static pressures at 6 weeks before the training, at the beginning of training, and after the end of 6 week-training. The first 6 weeks were used as a control period. Inspiratory muscle training consisted of breathing through Threshold inspiratory muscle trainer (IMT) at 30% of patients' maximal inspiratory pressures(MIP) for 15 minutes twice a day and the 'endurance time' was recorded weekly for an assessment of inspiratory muscle endurance. This study showed significant improvement of MIP and endurance time after the training in both ambulatory and wheelchair-bound patients. The amounts of improvement were greater in the patients with a better functional state and greater baseline forced vital capacity. We conclude that, in the early stages of DMD, inspiratory muscle training with pressure threshold device is more useful when the forced vital capacity is well preserved.