Paralysis or severe weakness of the diaphragm (the principal muscle of inspiration) is commonly associated with hypoventilation which may be profound during sleep, but the disorder is often unrecognized. The clinical sign of paradoxical (inward) movement of the abdominal wall during spontaneous inspiration in the supine posture may be overlooked, or the appearances at fluoroscopy may mislead, for it is not generally appreciated that descent of the diaphragm in early inspiration in these patients may be an entirely passive event. Measurement of transdiaphragmatic pressure is a simple, reliable and quantitative means of assessing diaphragm function, and will aid in the diagnosis of diaphragm paralysis which has important implications for clinical management.