The differentiation between lung abscess and empyema can be difficult, but has important therapeuticconsequences. Thoracostomy tube drainage is essential therapy for an empyema, whereas prolonged antibiotic therapyand postural drainage often suffice for a lung abscess. Conventional radiographic findings are usually relied onto help make correct indfferentiation between empyema and lung abscess, but overlying lung disease or unfavorablelocation of lesion often results in ambiguous findings. Although ultrasound has proved useful in differentiatingempyema from lung abscess, CT is best accurate diagnostic method. Authors reviewed chest CT of 50 cases(41empyemas, 9 lung abscesses)which were diagnosed by suegery or clinical background during the period from May 1980to June 1985 at Dongsan Medical Center, Keimyung University. The results were as follows: 1) Age and sexdistribution a) Empyema: The incidence was most common in the 6th and 7th decades. Male to female ratio was 3:1.b) Lung abscess: The incidence was most common in the 5th, 6th and 7th decades. Male to female ratio was 2:1. 2)Wall characteristics(empyema 32 cases, lung abscess 9 cases): The 9 cases of 41 empyemas had not defined theirwalls. a) Empyema had at least a part of their wall that was thin (81%), uniform width(84%), and smooth on bothmargins (more than 96%). b) The wall of lung abscess was thick (89%), and irregular margins(100%). 3) Separationof uniformly thickened visceral pleura from parietal pleura("split pleura" sign) was seen only in 68% of allempyemas. 4) Adjacent lung compression was seen only in 88% of all empyemas. 5) Chest wall angle: In 78% of allempyemas had obtuse or mixed angles, wherease in 85% of all lung abscesses had acute angle. 6) Shape oflesion:Empyema had variant shapes from round to crescent, however all lung abscesses had round or ovoid shape. 7)Size of lesion: In 85% of all empyemas had medium(41%) or large (44%) size, but the lung abscess had onlysmall(33%) or medium (67%) size. 8) Air in lesion was seen in 41% empyemas and in 89% lung abscesses. 9) Adjacentlung consolidation showed in 34% empyemas and in all lung abscesses. 10) Free pleural fluid was seen in 12%empyemas and in 89% lung abscesses. 11)Septated lesions were seen in 32% empyemas and in 56% lung abscesses. 12)Multiple lesions were seen in 44% empyemas and in 55% lung abscesses. 13) Mediastinal shifting was seen in 49%enpyemas and in 44% lung abscesses, but which all lung abscesses were coexisted with empyemas. 14) Pleuralcalcification was seen only in 5 cases(12%) of all empyemas. 15) Location of leison: Most(93%) of empyemas werelocated in posterolateral portion of hemithorax, and most (78%) of lung abscesses involved in right lower lobe.