PURPOSE: Extrapleural fat hypertrophy is thought to be associated with findings of chronic pleural or pulmonary parenchymal diseases, and more favours clinical course of benign disease rather than malignant disease which is rapidly progressive. Recently, however, we observed frequent extrapleural fat hypertrophy in CT scens of lung cencer patients, especially those with complicated ones. We studied the frequency of extrapleural fat hypertrophy and the relationship between the extrapleural fat hypertrophy and the complications of lung cancer. MATERIALS AND METHODS: We retrospectively analized the CT findings in 95 patients with primary lung cancer. 85 patients were confirmed pathologically and 10 patients were confirmed by clinical and radiological follow-up studies. RESULTS: Among the 95 patients, 79 patients were associated with more than one complications(Lung collapse, pneumonia, pulmonary tuberculosis., pleural reaction, pleural metastasis, empyema, chest wall invasion). Extrapleural fat hypertrophy was observed in 43(54%) patients with complication, and 2(13% ) patients without complication. The frequency of the extrapleural fat hypertrophy was 40%(14/35) in patients with only parenchymal complicated patients, 33%(2/6) in patients with only pleural complication, and 71%(27/38) in patients with both parenchymal and pleural complications. CONCLUSION: Extrapleural fat hypertrophy was more frequently observed in complicated lung cancer patients rather than non-complicated, and higher frequency of the extrapleural fat hypertrophy was noted in those with more than two complications. It was concluded that extrapleural fat hypertrophy in lung cancer patients was non-specific finding caused by associated complication. Its mechanism is thought to be chronic pleural irritation and negative thoracic pressure.