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The reported frequency of lung cancer in the setting of diffuse pulmonary fibrosis varies greatly, depending on the country of origin and the type of study. Most recent reports regarding diffuse pulmonary fibrosis in general and idiopathic pulmonary fibrosis in particular and lung cancers come from Japan; only a few clinical studies of this issue are available from other countries of the world, including the United States. The reported frequency ranges from 4.8% in the United States to 48.2% in Japan. The most frequent type of cancer is adenocarcinoma. Risk factors may include cigarette smoking, exposure to metal dusts, onset of idiopathic pulmonary fibrosis at an older age, and male predominance. Possible pathologic mechanisms are summarized. Given the very poor prognosis of idiopathic pulmonary fibrosis itself, with a mean survival of only 2.8 years, and that different diagnostic criteria were used in each study, it is likely that many of these studies are flawed because they evaluate lesions other than idiopathic pulmonary fibrosis. Thus, the frequency of lung cancer in idiopathic pulmonary fibrosis is still uncertain, and clearly requires follow-up of cohorts of clinically well-characterized patients, using standard diagnostic criteria for idiopathic pulmonary fibrosis. Finally, if the association between idiopathic pulmonary fibrosis and lung cancer is reconfirmed in these studies, the molecular and genetic mechanisms governing the development of lung cancer in this setting require additional study.