Fifty-seven patients with rheumatoid arthritis (RA) were treated in hospital for diffuse interstitial lung fibrosis. Although interstitial fibrosis (either on the basis of lung function tests or chest roentgenograms or both) is fairly common among patients with RA, according to this study interstitial fibrosis of sufficient extent or severity to warrant hospitalization was rare: incidence of hospitalization due to the lung disease in RA patients was one case per 3,500 patient-years. Eight patients had a largely reversible lung disease associated with drug treatment (gold, D-penicillamine or nitrofurantoin.) The remaining 49 had interstitial fibrosis of unknown cause. Causes for hospitalization were respiratory and general symptoms in 38, but infiltrations on routine chest roentgenographic examinations alone in eleven patients. Forty-five out of the 49 patients had crackles on auscultation. The most typical findings in lung function tests were restriction and a decreased diffusion capacity. These 49 patients showed a poor prognosis, with a median survival of 3.5 years and a five-year survival rate of 39 percent.