To define the clinical characteristics of multiple lung cancer (LC) associated with idiopathic pulmonary fibrosis (IPF), we reviewed 154 LC patients associated with IPF: 23 patients with synchronous multiple LC (IPF-multiple LC group) and 131 with single LC (IPF-single LC group), and these were compared with 4,931 patients with LC from 1975 to 1977 in Japan (whole LC group). In the IPF-single and IPF-multiple LC groups, most tumors were observed in male patients (91% and 96%), smokers (94% and 100%), and in peripheral regions of the lung (91% and 98%). The incidence of occurrence in the lower lobes, where a fibrotic shadow was prominent, was significantly higher in the IPF-LC groups (58% and 67%) than for the whole LC group (37%). The distribution of histologic types in the IPF-single LC group was similar to that of the whole LC group. However, the incidence of small cell carcinoma was significantly higher in the IPF-multiple LC group (33%) than for the IPF-single LC (14%) and whole LC (12%) groups. These results indicate that the features characteristic to synchronous multiple LC in patients with IPF are as follows: (1) male patients; (2) smokers; (3) small cell carcinoma histologic type; (4) lower lobes; and (5) peripheral type, all of which show a high rate of occurrence.