Noninvasive mechanical ventilation using bilevel positive pressure ventilation (BiPAP) has not been studied in acute respiratory failure caused by MG. Eleven episodes in nine patients were initially managed with BiPAP, and endotracheal intubation was avoided in seven of these trials. Presence of hypercapnia (PaCO2 greater than 50 mm Hg) at onset predicted BiPAP failure and subsequent intubation. Results of this preliminary study suggest that a trial of BiPAP may prevent intubation in patients with myasthenic crisis without overt hypercapnia.