Lung involvement was assessed in 30 consecutive patients with systemic lupus erythematosus (SLE), not selected by respiratory symptoms. Pulmonary function tests revealed a higher rate of abnormality than either clinical history or radiography. The single breath carbon monoxide diffusing capacity was below 80 per cent of the predicted value in 24 patients (80 per cent), and a reduced total lung capacity was present in 13 (43 per cent). There was a weak correlation between the severity of the functional defect and disease activity, assessed antinuclear factor and DNA binding. No correlation was found with serum complement of Clq precipitation. Since pulmonary fibrosis in SLE is uncommon it cannot account for the high frequency of abnormal findings, and the pathogenesis of the functional changes is probably multifactorial. In seven of the patients with the smallest lung volumes, measurements of static pressure volume curves and of maximum respiratory pressures indicated extrapulmonary volume restriction. In five of these patients, diaphragm function was specifically assessed and found to be grossly abnormal in four. The inability of the diaphragm to generate normal pressure may be due to either severe weakness or immobility following extensive pleural adhesions. The well recognized syndrome of "shrinking lungs" and high "sluggish" diaphragms with clear lung fields on radiography is probably due to dysfunction of the diaphragm rather than to primary intrapulmonary pathology.