Sixty-two patients with classical or definite RA who were considered suitable for disease-modifying drug therapy were entered into a prospective longitudinal study to determine the long-term effects of commonly used drugs such as gold, D-penicillamine and azathioprine on pulmonary function. Each patient had an assessment of pulmonary function, including spirometry, static lung volumes and diffusion tests, before starting drug therapy and at 6-monthly intervals during treatment. In those patients where drug therapy was discontinued, pulmonary functions were performed at withdrawal and 1 year later. Forty-six per cent of the patients in the 'gold group' and 21% in the 'penicillamine group' had developed a restrictive defect after 2 years. There was a significant reduction in vital capacity and transfer factor for carbon monoxide (P < 0.001) in both groups. A significant improvement in pulmonary function was found in the gold-treated patients (P < 0.05) a year after treatment was discontinued. This study shows that a significant proportion of rheumatoid patients on second-line drugs such as gold and penicillamine develop chronic progressive pulmonary damage indistinguishable from fibrosing alveolitis or 'rheumatoid lung'. Its recognition is important as the condition is asymptomatic, slowly progressive and appears to be reversible on withdrawal of the drug therapy in the majority of the patients.