The potential difficulties in distinguishing a pleural based parenchymal abscess from a loculated pleural ef-fusion or empyema on the basis of plain radiographs have been well documented. However chest ultrasonography can provide valuable informations about disease processes abutting the chest wall despite the physical limitations imposed by aerated lung and bony thorax that restrict the potential usefullness of sonography in chest diagnosis. Some authors proposed that chest CT can diagnose them more accurately and provides more valuable informations than ultrasonography can. we evaluated chest radiographs CT and ultrasonography of 36 patients with suspected pleural based lung abscess or empyema for the comparison of diagnostic modalities in the differentiation of these two disease entities. The results were as follows. 1. CT scan could accurately differentiate empyema from parenchymal lesions by observing wall characteristics split pleura adjacent lung compression and internal septations. 2, In 90% of patients with suspected lung abscess or empyema they could be differentiated with ultrasonography alone by indentifying internal septation echogenic layering along pleural surface and internal echo patterns, 3. Ultrasonography assisted plain radiography could also distinguish them as accurately as chest CT. 4. Chest ultrasonography was more valuable than CT in the evaluation of pleural thickening septation and comparmentalization of pleural cavity.