In order to investigate the etiology of usual interstitial pneumonia (UIP) and UIP with lung cancer (LC), autopsy findings in 18 cases of UIP with LC and 11 cases of uncomplicated UIP were clinicopathologically compared with the environmental factors of smoking habits and occupation. UIP with LC was highly correlated with smoking, especially heavy smoking and with occupations in which dust is inhaled, such as electrical installation and ceramic production, indicating that these environmental factors are important background factors in the complication of UIP with IC. Pathologic examination of cases of UIP with LC (6 squamous cell carcinomas, 5 small cell carcinomas, 4 adenocarcinomas, and 3 large cell carcinomas, 2 of which showed pulmonary double carcinoma revealing a slight correlation between fibrosis and primary site of LC and a slightly greater correlation of squamous cell carcinoma and small cell carcinoma to smoking habits and inhalation of dust. In terms of the correlation between UIP and LC among autopsy cases, the environmental factors proved to be more significant than the fibrotic findings. These environmental factors are thought to merit consideration as common predisposing factors in the development of LC and its complication with UIP.