To confirm diagnosis and to set proper therapeutic strategy, open lung biopsies were done in 57 patients who were suspected for diffuse interstitial lung disease from January 1985 to December 1994. Among them, 35 were male and 22 were female[M:F=l.6: 1 and mean age of the patients is 53.5$pm$ 2.3[24-81 years. Tissue for histologic studies were obtained from left lung in 33, from right lung in 24according to the distributions of the pathology. Preoperative diagnostic work-up`s were chest X-ray, CT[HRCT scan, sputum study, bronchoscopy[BAL, TBLB and PTNA and all of them were unsuccessful to confirm diagnosis. In comparison of pulmonary function tests between preoperative and postoperative values, there were no significant differences in FVC, FEV1, FEV1/FVC[p 0.05 but in AaDO2[p[0.05 . Postoperative complications including atelectasis, wound infection, pulmonary edema and respiratory insnfficiency, were shown in 5 cases[8.8% , and two of them were died of respiratory failure and sepsis[mortality rate 3.5% . Pathologic diagnosis was confirmed in 53 cases postoperatively but it was undetermined in 4[diagnostic yield rate 93.0% . In comparison between preoperative clinical diagnosis and postoperative pathologic diagnosis, new diagnosis were made in 17 cases[29.8% and preoperative tentative diagnosis were confirmed histologically in 36 cases[63.2% . In 4 cases[7.0% , however, diagnoses were not confirmed after biopsies. Therapeutic plans were reset in 46 cases[80.7% in accordance with the final diagnosis.In conclusion, open lung biopsy is recommended for a specific diagnosis and proper therapeutic plan in diffuse interstitial lung diseases because of its high diagnostic yield Irate and it`s relatively low morbidity and mortality rate in these tompromised patents.