COPD is an independent risk factor for lung cancer. There is emerging evidence that chronic inflammation may play a significant role in the pathogenesis of lung cancer as a tumor promoter. Cigarette smoke exponentially up-regulates the production of cytokines. After stopping smoking, the risk of lung cancer remains increased in patients with COPD. We report 3 patients with COPD in whom lung cancer was detected within 16 months after smoking cessation. All were outpatients of our hospital, and participated in a program for smoking cessation according to their doctor's advice. Two successfully stopped smoking, and the other was in the program for smoking cessation. It was difficult to detect lung cancers on chest X-ray films 8-11 months previously. Periodic medical examination chest X-ray films revealed lung cancers 2-16 months after smoking cessation. Heavy smokers, especially COPD patients, have a high risk of developing lung cancer at the start of smoking cessation, and therefore have a high risk of contracting lung cancer even after halting smoking. Generally, most patients do not visit hospitals for the purpose of only smoking cessation after successfully halting smoking. Nevertheless, it is necessary for previously heavy smokers, especially COPD patients, to undergo repeated careful medical examination to detect lung cancer.