Thirty patients with breathlessness and diaphragm weakness were studied by measuring transdiaphragmatic pressures during maximal inspirations to total lung capacity, maximal static inspiratory efforts from residual volume, and maximal sniffs from functional residual capacity. Maximal static respiratory mouth pressures were also recorded, and rib cage and abdominal movements were monitored with pairs of magnetometers. Sniff transdiaphragmatic pressure was abnormally low in all patients and was correlated with transdiaphragmatic pressure during other maneuvers, and with maximal static inspiratory mouth pressures. There was no relationship between the severity of dyspnea and transdiaphragmatic pressure in the group as a whole. The weakest patients had orthopnea and paradoxical inward inspiratory motion of the anterior abdominal wall; measurements suggested that at least 30 cm H2O transdiaphragmatic pressure was required to overcome the hydrostatic pressure of the abdominal contents. By contrast, patients with mild diaphragm weakness had neither orthopnea nor abdominal paradox. Thus, patients with breathlessness and diaphragm dysfunction may have varying degrees of diaphragm weakness that may be difficult to detect clinically; the diagnosis and quantification of diaphragm weakness requires the measurement of transdiaphragmatic pressure.