BACKGROUND: It is well known that the prevalence of lung cancer is higher in idiopathic pulmonary fibrosis(IPF) patients than in the general population. This high prevalence is explained by the concept of 'scar carcinoma'. There have been several reports on the prevalence of histologic type of lung cancer in IPF with conflicting results. Despite of the high smoker rate in almost all previous reports, none considered the smoking history of patients. Therefore we performed a separate studies on fibrosis associated lung cancer and smoking associated lung cancer. The purpose of this study is to investigate the proportion of lung cancer in IPF that is fibrosis assiciated and to determine the most common histologic type in fibrosis associated lung cancer in IPF. METHODS: A retrospective review of medical records and radilolgic studies was performed for cases of lung cancer with IPF. We investigated smoking history, sequence of diagnosis of lung cancer and IPF, histologic type of lung cancer and the cancer location, especially whether the location is associated with fibrosis. To evaluate the proportion of fibrous associated lung cancer, the lung cancer in IPF were categorized according to the presence of fibrosis at cancer location. RESULTS: Fifty seven patients were subjects for this analysis. Six(11%) cases were diagnosed as lung cancer during follow-up for IPF, and both diseases were diagnosed simultaneously in the others. Ninety four percent of patients were smokers and the average smoking amount was 47.1±21.9 pack-year. Among the patients with IPF and lung cancer, 42(80.8%) cases were considered as 'fibrosis associated'. The remainder was 'not fibrosis associated' and probably was due to smoking etc. Although the most frequent histologic type was squamous cell carcinoma as a whole, adenocarcinoma was the prominent histologic type in 'fibrosis associated lung cancer.' CONCLUSION: Considering the proportion of 'fibrosis not associated lung cancer' in the patients with IPF and lung cancer, significant proportion of lung cancer in IPF may not be fibrosis induced. This may influence the distribution of histologic type of lung cancer in IPF.