The role of inspiratory muscle dysfunction in lung volume restriction and unexplained dyspnea was studied in 16 consecutive patients with systemic lupus erythematosus. Maximal mouth inspiratory pressure (PIM) and maximal transdiaphragmatic pressure (Pdi max) were measured. Pdi and its components were determined during quiet breathing. No significant association was found between the activity of the disease, several serologic markers, and the inspiratory muscle dysfunction. No specific anti-skeletal muscle antibody was found in these patients. Significant correlations were found between the degree of dyspnea and PIM (r = -0.69, P less than 0.01) and Pdi max (r = -0.75, P less than 0.001); however, dyspnea did not correlate with specific lung compliance. Vital capacity correlated significantly with the degree of dyspnea (r = -0.813, P less than 0.001) and with Pdi max (r = 0.544, P less than 0.05). No correlation was found between vital capacity and specific lung compliance. We conclude that inspiratory muscle dysfunction can be an important mechanism in the pathogenesis of the lung volume restriction and dyspnea in patients with systemic lupus erythematosus.