Computed tomography (CT) was performed in 73 patients with pulmonary infiltrative changes of unknown etiology. Their preliminary clinical and X-ray studies suggested protracted pneumonia in 46.6%, malignant neoplasms in 27.4%, tuberculosis in 16.1%, and pyesis in 9.9%. The preliminary clinical and X-ray conclusion fully agreed with the verified diagnosis only in 31.5% of patients. The accuracy of CT in assessing infiltrative changes was much higher and ran to 89.1%. According to CT findings, nontumor diseases were found in 56 (76.7%) patients, malignant tumors in 17 (23.3%). Among inflammatory processes there were prevalent pyesis 23.3%), pulmonary tuberculosis (20.7%) and pneumonias (19.2%). In the group of 17 patients with neoplasms, 8 were found to have central cancer with hypoventilation of a lung portion, 4 had peripheral cancer, and 5 presented with bronchoalveolar carcinoma. The use of high-performance CT is of great importance.