PURPOSE: It is well known that the incidence of lung cancer is high in patients with idiopathic pulmonary fibrosis(IPF). We analyzed the CT features of lung cancer associated with IPF. MATERIALS AND METHODS: Retrospective analysis was performed in 23 patients with lung cancer(24 lung cancers) associated with IPF. The diagnosis of IPF was made by clinical and CT findings, and lung cancer was confirmed pathologically. We dividedthe location of lung cancer by lobar distribution and central or peripheral lung zone, and measured the size ofmass. We classified the mediastinal lymph node enlargement by American Thoracic Society(ATS) mapping scheme. We evaluated the CT pattern of IPF. RESULTS: The subjects consisted of 6 cases of small cell carcinoma and 18 cases of non-small cell lung cancer. Non-small cell lung cancer included 8 cases of squamous cell carcinoma, 4 cases of adenocarcinoma, 2 cases of large cell carcinoma, 3 cases of poorly differenciated carcinoma, and 1 case of unclassified malignant neoplosm. The lung cancers were located in the right upper lobe in 5 cases, left upper lobein 6 cases, right middle lobe in 1 case, right lower lobe in 9 cases, and left lower lobe in 3 cases. Twenty cancers(85%) were located in the peripheral lung zone. Eighteen cancers(73%) were surrounded by fibrotic lung. The size of the mass ranged from 1 to 12cm, and in 12 cases it was below 3cm in diameter. Mediastinal lymph nodes were enlarged in 22 cases(92%) and classified as N2 or N3 in 15 cases out of 18 non-small cell lung cancers. CT patterns of underlying IPF were honey-combing in 18 patients(78%) and mixed honey-combing and ground-glassopacity in 5 patients(22%). CONCLUSION: The lung cancer associated with IPF shows variable cell types. Most ofthe lung cancers were located peripherally, surrounded by end-stage fibrosis, and were associated with mediastinallymph node enlargement.