Since several reports were published about CT differentiation of peridiaphragmatic fluid collection using 4 useful signs-diaphragm displaced crus bare area and interface signs. Trans-verse CT scans of 20 patients with abnormal diaphragmatic position due to large intrathoracic or intraabdominal lesion were analysed on the basis of those sings. Difficulties were encountered with differentiation when laterally located lesions did not extend to as far medially as crus and when diaphragmatic stripe could not be distinguished from thickened pleura or adjacent wall of lesions. As a result limited cases can be adequately assessed by diaphragm or displaced crus sign. Furthermore bare area and interface signs seemed to be not useful at all. However relationship between caudal tip of lesions and thoracoabdominal wall was always constant in each thoracic or abdominal lesions. All of intrathoracic masses or empyemas were attached to thoracic wall displacing properitoneal and perirenal fat medially or inferiorly. By contraries all of intraabdominal mases were seperated from abdominal wall displacing proper-itoneal fat or peritoneum laterally. The key to accurate localization seemed to be identification of such relationship.