To study the frequency of the underlying disease of the diffuse pulmonary infiltrates in korea, we ansalyzed retrospectively 982 patients who were seen at nine university hospitals and one general hospital in Seoul area. The results are folloing: 1) Among the total 982 patients, 490 patients were male and 492 patients were female. The mean age was 44.3 years. 2) The most common etiology was milliary tuberculosis (38%), which was followed by, idopathic pulmonary fibrosis (27%), pulmonary fibrosis associated with collagen-vascular disease (15%), and diffuse pulmonary infiltrates by malignancy (10%). 3) Amon the connective tissue disease which was accompanied by the interstitial lung disease, rheumatoid arthritis was the most common disease (43%), systemic lupus erythematosus was the 2nd (28%), and progressive systemic sclerosis was the 3re (16%). 4) Among 101 cases of malignant disease, lung was the most frequent primary site (31%), which was followed by stomach (28%), thyroid (16%), and breast (6%). 5) For the diagnosis of the underlying disease of pulmonary infiltrates, the transbronchial lung biopsy was performed in 21% of the patients and open lung biopsy was done in 7%.