CT plays an important role in evaluating mediastinal lymphadenopathy especially in the staging of lung cancer. However its role has been confined to the detectibility of pathologic lymph nodes by using the size criteria Still seldom effort has been made on morphologic analysis of the diseased lymph nodes. We analyzed post-enhance CT scans of 64 pathologically proven patients who had mediastinal lym-phadenopathy with special reference to nodal architecture 24 of the 41 patients with lung cancer and 15 of the 17 patients with tuberculous mediastinal lymphadenitis showed intranodal necrotic low density. Not only tuberculous lymph nodes showed higher incidence but also showed higher conspicuity of the necrotic low den-sity than that of the metastatic nodes from lung cancer. Conspicuity of internal low density in tuberculous nodes was due to significantly higher density of enhanced rim (121.7$pm$18.7 HU) than that of metastatic nodes(91.2 $pm$23.4 HU) (P<0.01) As the node size increases incidences of necrotic low density increased both in tuberculous and metastatic nodes but the latter steeper. Analysis of the location of the diseased node did not show any significant difference between tuberculous and metastatic nodes. If there is coexistence of primary lung mass the possibility of lung cancer was markedly elevated than that of tuverculosis and the internal low density in lung mass shows some tehdency of coexistence of central low density within metastatic lymph nodes.