In experimental animals, conditions which drastically decrease cardiac output may reduce the strength and endurance of respiratory muscles leading to hypercapnic respiratory failure. Because patients with chronic CHF have reduced cardiac output and vital capacity (FVC), we measured PImax and PEmax and maximal handgrip force in 16 patients with CHF and 18 AMNs. The patients with CHF had a mean left ventricular ejection fraction of 26 +/- 7 percent. Maximal respiratory pressures were significantly reduced; group mean values (+/- SD) for PImax at FRC were 41.4 +/- 5.6 cm H2O (CHF) and 102.1 +/- 27.4 cm H2O (AMN) (p less than 0.001), with PImax values in five patients with CHF as low as 20 to 30 cm H2O. In most patients, PEmax was comparably reduced. Handgrip force was less dramatically reduced, suggesting selective respiratory muscle weakness. Possible explanations include reduction in respiratory muscle blood flow or generalized muscular atrophy and weakness related to cardiac cachexia.