Functional alterations in the inspiratory muscles were evaluated in patients receiving corticosteroids for diseases other than respiratory. Inspiratory muscle strength, as expressed by the maximal inspiratory mouth pressure (PImax), and inspiratory muscle endurance (PmPeak/PImax), using a pressure threshold breathing device, were evaluated in eight patients with normal pulmonary and inspiratory muscle functions (two patients with rapidly progressive glomerulonephritis, two with glomerulonephritis with minimal changes, two with idiopathic thrombocytopenic purpura, and two with subacute thyroiditis). There was a gradual decrease in both inspiratory muscle strength and endurance following corticosteroid administration. After 8 weeks of treatment PmPeak/PImax decreased from 84.4 +/- 2.4 to 67.9 +/- 3.1 percent (p < 0.001), while inspiratory muscle strength dropped from 126.9 +/- 9.6 to 86.5 +/- 7.4 cm H2O (p < 0.005). Gradual steroid dosage tapering resulted in marked improvement in both strength and endurance; the inspiratory muscle strength rose significantly to 112.2 +/- 8.1 cm H2O (p < 0.0005) when steroid treatment was stopped, and even more significantly 6 months later (to 123.1 +/- 8.1 cm H2O [p < 0.0001]), and the PmPeak/PImax rose to 60.6 +/- 3.4 percent (p < 0.001) and to 74.7 +/- 3.2 percent (p < 0.0001), respectively. We conclude that corticosteroids have a significant deteriorating effect on respiratory muscle function in humans. This weakness is reversible while tapering steroid dosage. Steroid therapy should be reconsidered in patients with underlying lung disease.