According to the National Vital Statistics data, age-standardized mortality rates (ASRs) of lung cancer have shown slightly declining trends in Japan for both men and women. In order to evaluate whether this tendency will continue, a Bayesian age-period-cohort (APC) model was applied using the National Vital Statistics data from 1952 to 2001. In the projection, a Gaussian autoregressive prior model was applied to smooth age, period, and cohort effects from its 2 immediate predecessors by extrapolation. Posterior distributions from which we drew inferences on mortality rates were derived from 15,000 iterations using 5000 burn-in iterations. We defined the median of the iterated values as the overall summary mortality rate of the iterated results. Our results suggest that the number of deaths due to lung cancer will double for men and women during the next 3 decades due to the aging of the baby-boomer generation (individuals who were born between 1947 and 1951). Currently declining trends in some age groups will reverse and start to increase again in the next decades. However, for recent birth cohorts, the results of the projection varied according to whether the data set included early age group mortality or not. Lung cancer mortality in the future depends on the risk factors engaged in by today's young people, especially smoking. Strong promotion of anti-smoking measures and careful surveillance for lung cancer are needed.