We studied six patients with isolated bilateral paralysis or severe weakness of the diaphragm, present for 2 to 60 months (mean = 25), to document the clinical and respiratory sequelae of the condition. Severe diaphragm dysfunction was confirmed by the demonstration of the very low maximal transdiaphragmatic pressure (Pdi) generated by either a sniff (13 +/- 6 cm H2O, normal 148 +/- 24) or a static inspiration (11 +/- 8, normal 108 +/- 30) and during bilateral phrenic nerve stimulation (0.8 +/- 2.0, normal 22 +/- 4). Resting arterial blood gases were normal (SaO2 = 95 to 97%) and no oxygen desaturation occurred during maximal exercise on a treadmill. Maximum voluntary ventilation was low and related to PImax (r = 0.89). Overnight sleep monitoring showed that time spent in rapid eye movement sleep was normal (mean 55 +/- 36 min, range 26 to 117 min). Mean maximum increment in transcutaneous CO2 was within normal limits (6 +/- 2 mm Hg, range 3 to 9 mm Hg). Three patients had occasional brief episodes of oxygen desaturation (mean maximal decrease 13 +/- 10%, range 2 to 27%); however, only two of these spent a measurable proportion of total sleep time (TST) with an SaO2 of less than 80% (1% and 3% TST, respectively). No patient has developed any symptoms of nocturnal hypoventilation or chronic respiratory failure during periods of observation of up to five yr. We conclude that bilateral paralysis or very severe weakness of the diaphragm does not of itself lead to respiratory failure unless weakness of other respiratory muscles is present.