Impaired cough that results in ineffective airway secretion clearance is an important contributor to pulmonary complications in patients with neuromuscular weakness including spinal cord injury. Mechanical insufflation-exsufflation (MI-E) is a respiratory aid used by patients with weak respiratory muscles to increase cough peak flows and improve cough effectiveness. Relative contraindications to MI-E are said to include susceptibility to pneumothorax, but the association of pneumothorax with MI-E use has never before been described. We report two cases of pneumothorax in patients with respiratory muscle weakness associated with daily use of MI-E: one was a 58-yr-old male with C4 ASIA C tetraplegia, and the other was a 26-yr-old male with Duchenne muscular dystrophy. Both patients also used positive-pressure ventilatory assistance. Although seemingly rare in this patient population, ventilator users also using MI-E who have increasing dyspnea or who require increasing positive inspiratory pressures when using noninvasive ventilation should be evaluated for pneumothorax.