The mass screening of lung cancer has been started with financial support of the Japanese Government from 1987. It should be emphasized, however, that mass screening programs of any kind have to be evaluated by means of benefit-risk analysis and cost-effectiveness analysis. This is the first report on the benefit-risk analysis for mass screening program of lung cancer in Japan. The benefit of the lung cancer mass screening is defined as a net elongation of average life expectancy due to early detection of the cancer. It is calculated as a function of age and sex. While, the risk of the screening program is defined as a net shortage of average life expectancy due to radiation carcinogenesis of leukemia and lung cancer. In the case of radiation carcinogenesis, latent time and plateau period are considered in the calculation of the risk. Since the benefit increases with age and the risk decreases with age for both sexes, one can obtain a certain age at which the benefit and the risk cross. Assuming dose equivalent of lung of 1 mSv and risk coefficient of 15.1 X 10(-3) Sv-1, the crossing ages of men and women are about 42 y.o. and 47 y.o. respectively. We consider that these ages are rather high when chest radiograph is to be used as a screening test. It is recommended that the dose equivalent of lung should be lowered to 0.1 mSv if the mass screening of lung cancer is to be performed.