We investigated lung cancer in 99 patients with idiopathic pulmonary fibrosis (IPF). Lung cancer was found in 31 (31.3%) of 99 patients with IPF. Most (87.9%) tumors, including squamous cell carcinoma, were observed in the peripheral region of the lung, whereas the distribution of histologic types of cancers was similar to that seen in ordinary lung cancer. Peripheral tumors were frequently seen in the lower lobe, where fibrotic shadow was prominent. However, the severity of fibrosis was not related with the prevalence or histologic type of lung cancer. Two-thirds of IPF patients having a smoking history of over 40 years developed lung cancer. When compared with nonsmoking IPF control subjects, the relative risk of smoking in IPF patients was 3.5, identical with that reported for smokers in the general population. We suggest that smoking in patients with IPF is an additive risk factor for the development of lung cancer. We also speculate that the high prevalence of peripheral squamous cell carcinoma might be associated with cigarette smoking.