To determine the prevalence of pulmonary dysfunction in lupus erythematosus, 24 patients with systemic lupus erythematosus (SLE) and 5 patients with discoid lupus erythematosus (DLE) were studied. Diffusing capacity for carbon monoxide was abnormal in 17 (71 percent) SLE patients. A restrictive ventilatory defect was present in 6 (25 percent) and arterial hypoxemia in 4 of 23 (17 percent). The mean ratio of forced expiratory volume in one second to forced vital capacity (FVC) was 83 percent. To test for the presence of small airways disease, maximum expiratory flow rate at 50 percent of FVC was measured on air and on an 80 percent helium-20 percent oxygen mixture. Ten patients (5 smokers and 5 nonsmokers) with SLE were nonresponders to helium suggesting small airways disease. Pulmonary dysfunction was present in 90 percent (9/10) of SLE patients with a previous history of pleuritis and/or pneumonitis, and in 71 percent (10/14) without respiratory symptoms or history of lung disease and with a normal chest radiograph. Pulmonary function tests were normal in DLE patients except for an abnormal response to helium and/or mild arterial hypoxemia in two patients, all of whom were smokers. These data indicate that there is a high prevalence of pulmonary function abnormalities in SLE including patients without clinically evident pleuropulmonary disease.