Using a retrospective analysis of 161 patients with amyotrophic lateral sclerosis (ALS) from the Western ALS study group (WALS) database, the sensitivity of maximal inspiratory pressure (MIP)< -60 cm H(2)O and forced vital capacity (FVC)< 50% as US Medicare thresholds for initiating non-invasive ventilation (NIV) were compared. Sixty-five per cent of patients at enrollment met the MIP criterion, compared with only 8% of patients who met the FVC criterion. There were no cases in which FVC< 50% antedated MIP< -60 cm H(2)O. The longitudinal data showed that patients reached the MIP criterion 4 to 6.5 months earlier than the FVC criterion. For patients with clinical signs and symptoms needing treatment with NIV, a MIP< -60 cm H(2)O allows US clinicians to obtain non-invasive ventilatory support for patients earlier than if using the FVC criterion alone.