We present our experience with 27 patients with symptoms of alveolar hypoventilation, a precursor to respiratory failure, to demonstrate variability in symptoms, physiologic status and outcome of intervention. They represent 27 consecutive patients who tolerated NIPPV for more that 4 hours per 24-h period for more than 2 weeks. All patients received neurological consultation, electromyography and met criteria for ALS according to El Escorial diagnostic criteria. To assess respiratory status, spirometry was measured in sitting and when possible, in the supine positions. Resting arterial blood gases were available in 22 patients. Orthopnea was the most common symptom at the time of institution of NIPPV. No correlation existed between age at institution of NIPPV, duration of effective use of this technology or vital capacity and duration of effective use of NIPPV. The lack of correlation between vital capacity at the institution of NIPPV and duration of its effectiveness suggest that more sensitive indicators for the onset of alveolar hypoventilation must be defined, particularly since the principal benefit from its use is relief of symptoms of alveolar hypoventilation.