Open lung biopsy still has important roles for the marking of diagnosis of diffuse infiltrative lung disease even though transbronchial bronchoscopic lung biopsy and percutaneous needle aspiration biopsy gain popularity nowadays. This is clinical retrospective review of the 56 patients with diffuse infiltrative lung disease undergoing open lung biopsy by minithoracotomy from 1984 to Dec. 1992 in the Department of Thoracic & Cardiovascular Surgery of Catholic University Medical College. 27 men and 29 women, aged 17 to 73 year [mean 49 year , were enrolled & divided into 2 groups;Group A consisted of patients with immunocompromised state [n=19 , Group B patients with non-immunocompromised state[n=38 . Pathologic diagnosis was made in 54 cases[96.4% of these two groups and as follows: infectious; 12 patients[21.4% , Neoplastic; 10 patients[17.9% , granulomatous; 4 patients[7.1% , interstitial pneumonia; 12 patients[21.4% , Pulmonary fibrosis; 8 patients[14.3% , others; 3 patients[5.4% , nonspecific; 5 patients[8.9% , and undetermined; 2 patients[3.6% . Therapeutic plans were changed in 39 patients[69.6% after taking of tissue diagnosis by open lung biopsy. Group B has higher incidence of infectious diseases and change of therapeutic plan than the Group A. The postoperative complications developed in 8 cases[14.3% ,and there is no difference of incidence between the 2 groups. 4 patients belongs to group A, died of respiratory distress syndrome [2 and sepsis [2 which were not related with open lung biopsy procedure. In conclusion, open lung biopsy is a reliable method to obtain a diagnosis in diffuse pulmonary infiltrates and can be performed safely, even in acutely ill, immunosuppressed patients.