During 16 years (1958-1974) 327 open lung biopsies were carried out in the Tuberculosis and Respiratory Diseases Research Institute, the 2md and 3rd Surgical Clinic of Charles University in Prague. A modification of the original Klassen's surgical technique with axillary or submammary approach was used. The patients (160 males, 167 women) were referred for biopsy because of various forms of diffuse pulmonary lesions without established diagnosis. The age of the patients varied from 6-72 years, 27 of them, aged over 60, were operated on nothwithstanding their compromised cardiopulmonary function. In 252 (79%) the definite diagnosis with characteristic histology was obtained, and in 70 (21%) only pulmonary fibrosis without exact specification was evaluated. The definite histologic findings were as follows: 102 (31%) diffuse interstitial pulmonary fibrosis (Hamman-Rich), 32 (10%) pulmonary lesions in systemic cardiopulmonary or bronchopulmonary diseases (collagenosis, eosinophilic and other granulomas, Wegener, hemosiderosis, etc.), 42 (13%) pulmonary sarcoidosis, 12 (4%) pulmonary tuberculosis, 45 (14%) pneumoconiosis, mostly silicosis, 24 (7%) neoplastic lesions. Post-operative morbidity of minor importance was 13,5%, mortality 0,3%: One 56 years old woman died 5 days following operation of cardiopulmonary insufficiency. This was an incorrect indication for biopsy based on inadequate evaluation of the patient's initial functional status. The open lung biopsy obtained by minimal thoracotomy is a valuable and well tolerated diagnostic method in cases where diagnosis cannot be obtained with certainty on the base of results of usual investigation procedures.