It is widely known that patients with idiopathic pulmonary fibrosis (IPF) are frequently associated with lung cancer. Although a complication with lung cancer is an important prognostic factor for IPF, standard treatments for lung cancer cannot be given because of IPF. Especially, the administration of many anticancer agents is limited by a complication with IPF, which is recognized as a risk factor for the development of fatal lung injury in cancer chemotherapy. Epidemiological studies reveal that cigarette smoking and occupational and environmental exposure to toxic substances are common risk factors for both IPF and lung cancer. It has been assumed that metaplasia in fibrous lesions is pathologically a precancerous lesion, but it is necessary to prove several genetic abnormalities in the process of carcinogenesis in order to clarify that. Currently, several genetic abnormalities in IPF, including in p53, K-ras, FHIT and transforming growth factor (TGF)-beta 1 type II receptor, have been reported.