An assessment of pulmonary mechanics revealed weakness of inspiratory and expiratory muscles as cause of the restrictive ventilatory defect (vital capacity: 47 +/- 10 percent predicted) in seven (ages 33 to 62 years) out of 26 consecutive patients with systemic lupus erythematosus (SLE). Maximal inspiratory transdiaphragmatic pressures were reduced mainly due to the markedly increased (more positive) esophageal pressures. During maximal expiratory efforts, esophageal and gastric pressures were grossly decreased. These abnormalities probably may be considered part of a more generalized, yet subclinical muscle disorder due to the SLE with, however, predominant involvement of the respiratory muscles. The abnormalities were not related to the use of corticosteroids or generalized inanition. The static expiratory lung compliance was reduced in all patients (55 +/- 18 percent predicted), but normalized immediately following passive inflation of the lungs to transpulmonary pressures of more than 30 cm H2O in two of the three subjects tested. The volume restriction was not progressive over a period of 38.5 patient-years.