Pneumothorax (PTX) can be a serious complication of noninvasive positive pressure ventilation (NPPV). However, the exact incidence or background of PTX during a following NPPV therapy remains unclear. In our hospital, we examined patients with PTX related to NPPV therapy and compared them with non-PTX patients over a period of 7 years. Until 2004, 5 (5 patients) of 72 episodes (63 patients) of NPPV in acute chronic respiratory failure (RF) were accompanied by PTX (incidence, 6.9 percent). The 5 patients consisted of 4 men and 1 woman (mean age, 78.4 years and range, 74-84 years). The underlying diseases were interstitial lung diseases (ILDs) in 3 patients, chronic obstructive pulmonary disease in 1, and pneumonia in 1. The stage of RF in the 5 patients was acute in 4 and chronic in 1, and the duration between the initiation of NPPV and occurrence of PTX was within 10 days in 3 patients, 11-20 days in 1, and more than 20 days in 1 (mean duration, 15.6 days). The mean inspiratory positive airway pressure/expiratory positive airway pressure (IPAP/EPAP) of the 5 patients (10.6/4.1 cm H2O) was similar to that of 58 non-PTX patients (9.9/4.0 cm H2O). With regard to underlying diseases, the ratio of ILDs was higher in PTX patients (60%) than in non-PTX patients (8.6%) (p < 0.02). Patients at a high risk of PTX should be treated carefully with NPPV. Further, the therapy should be appropriately managed, especially in the early stage of NPPV initiation.