PURPOSE: The incidence of lung cancer in patients with idiopathic pulmonary fibrosis(IPF) is higher than that of general population. To evaluate the frequency and CT findings of lung cancer associated with idiopathic pulmonary fibrosis, we analyzed 19 patients with lung cancer associated with idiopathic pulmonary fibrosis. MATERIALS AND METHODS: We analyzed retrospectively 19 patients with histologically confirmed lung cancer out of 208 patients diagnosed as IPF either by CT and clinical findings(n=188) or histologically(n=20). All 19 patients were male, aged 40--85 years (mean 66 years). Scanning techniques were conventional CT in 12 patients, HRCT in 1 patient and both conventional CT and HRCT in 6 patients. We analyzed the CT patterns of lung cancer and IPF, locations of the tumor and histologic types of lung cancer. RESULTS: The incidence of lung cancer in patients with idiopathic pulmonary fibrosis was 9.1%(19/208). In 11 of 19 patients, CT findings of lung cancer were ill-defined consolidation-like mass. Lung cancer was located mainly in lower lobes(right lower Iobe;10/19, left lower Iobe;5/19) and at the periphery(12/19). Histologically, squamous cell carcinoma was the most common cell type (11/19). CONCLUSION: The incidence of lung cancer in patients with idiopathic pulmonary fibrosis was much higher than that of general population. Typical CT findings of lung cancer were predominantly ill-defined consolidation like mass at the peripheral lung portion which is the [ocatiaon where the most advanced fibrosis occur.