 Amyotrophic lateral sclerosis ALS is an untreatable neurodegenerative disorder characterized by the progressive loss of motor neuron function. Respiratory failure is the most common cause of death in persons with ALS. Treatment of respiratory muscle weakness with nocturnal noninvasive positive pressure ventilation NIPPV when forced vital capacity FVC is less than 50 percentbut prior to the development of respiratory failurehas prolonged survival in observational studies. Despite the association of NIPPV use and survival, it is unknown whether earlier NIPPV treatment will benefit people with ALS. Also, no placebo-controlled studies of NIPPV treatment and people with ALS have been conducted. 