The number of deaths from lung cancer is almost 40,000 a year in Japan (the rate is 45 per 100,000 for men and 12 per 100,000 for women). The incidence of lung cancer was 64 and 18 per 100,000 for men and women, respectively, in Osaka in 1987-89. Both the mortality and incidence have been increasing over the last few decades. However, the rates are lower than those in other ethnic groups and in America and other Western countries. The relationship between active smoking and lung cancer is well established, but further epidemiologic and basic studies on passive smoking and lung cancer are required. The basic epidemiologic methods of cohort studies and case-control studies are presented, and the basic concepts of clinical epidemiology are also discussed.