Several studies have investigated lung function in patients with rheumatoid arthritis but have reached different conclusions. The main discrepancy has been between airways disease reported in 38-65 per cent of patients and interstitial pulmonary disease reported in 30-41 per cent. These variable results have probably arisen because specific lung disorders have often been diagnosed on the basis of non-specific tests of lung function which, when considered in isolation, are subject to different interpretations. We adopted a combined epidemiological and clinical approach to investigate lung function and respiratory symptoms in patients with rheumatoid arthritis. Epidemiological data showed that rheumatoid arthritis is associated with a mild restrictive lung defect with reductions in mean FEV1 and FVC of 0.26 l and 0.29 l respectively and a normal FEV1/FVC ratio. The reduction in mean maximum mid-expiratory flow rate of 0.34 l/s could be explained on the basis of lung restriction and there was no evidence of widespread airways dysfunction other than that which could be explained by cigarette smoking. The clinical study showed that abnormal lung function tests in individual patients were caused by a heterogeneous group of conditions which are frequently caused, or exacerbated, by cigarette smoking. Cigarette smoking, and not the rheumatoid process, was the most frequent cause of abnormal lung function in rheumatoid arthritis.