In order to detect the interstitial pulmonary disease of rheumatoid arthritis (RA), we studied 91 patients with RA and compared the appearances of high resolution CT with those of plain chest X-ray film. We found 43 cases of interstitial pneumonitis on CT. We classified the CT and X-ray appearances as 4 groups (0-3) based on the degrees of pulmonary lesions. The both classifications accorded only in 46 cases (51%). In 46 cases who were not detected any changes by plain X-ray film, 17 were detected interstitial changes by CT. As to the early interstitial changes, the detectability of CT was superior to that of plain X-ray film.