In Japan, the number of lung cancer deaths has increased continuously since 1950, and in 1993 it was the leading cause of cancer deaths in males. This increase has been especially noteworthy among older people. In contrast, lung cancer deaths have started to decrease in the UK and the US, where anti-smoking activities have been strong since the 1970s. Obviously, smoking is the most important risk factor for lung cancer. Smoking can explain almost all case of squamous and small cell carcinoma. For adenocarcinoma, however, major unknown risk factors remain to be defined. Because the effectiveness of lung cancer screening is limited, it is necessary to integrate all available methods, including newly developed techniques, to prevent lung cancer deaths.