To evaluate the impact of smoking cessation on individuals and populations, we examined the decrease in risk of lung cancer death in male ex-smokers by age at quitting by pooling the data from three large-scale cohort studies in Japan. For simplicity, subjects were limited to male never smokers and former or current smokers who started smoking at ages 18-22 years, and 110,002 men aged 40-79 years at baseline were included. During the mean follow-up of 8.5 years, 968 men died from lung cancer. The mortality rate ratio compared to current smokers decreased with increasing attained age in men who stopped smoking before age 70 years. Among men who quit in their fifties, the cohort-adjusted mortality rate ratios (95% confidence interval) were 0.57 (0.40-0.82), 0.44 (0.29-0.66) and 0.36 (0.13-1.00) at attained ages 60-69, 70-79 and 80-89 years, respectively. The corresponding figures for those who quit in their sixties were 0.81 (0.44-1.48), 0.60 (0.43-0.82) and 0.43 (0.21-0.86). Overall, the mortality rate ratio for current smokers, relative to non-smokers, was 4.71 (95% confidence interval 3.76-5.89) and those for ex-smokers who had quit smoking 0-4, 5-9, 10-14, 15-19, 20-24 and >or=25 years before were 3.99 (2.97-5.35), 2.55 (1.80-3.62), 1.87 (1.23-2.85), 1.21 (0.66-2.22), 0.76 (0.33-1.75) and 0.67 (0.34-1.32), respectively. Although earlier cessation of smoking generally resulted in a lower rate of lung cancer mortality in each group of attained age, the absolute mortality rate decreased appreciably after stopping smoking even in men who quit at ages 60-69 years.