The relationship of physical activity to the development of definite coronary heart disease was examined separately in middle-aged (45-64 years) and elderly men (65-69 years) participating in the Honolulu Heart Program. After 12 years of follow-up, results indicate that increased levels of physical activity reported at study entry were inversely related to the risk of definite coronary heart disease in both age groups. In particular, among those aged 45 to 64 years, the rate of definite coronary heart disease in men who led active life styles was 30 per cent lower than the rate experienced by those who were less active (relative risk, 0.69; 95% confidence interval, 0.53, 0.88). In those older than 64 years, the rate of definite coronary heart disease in active men was less than half the rate experienced by those who led more sedentary life styles (relative risk, 0.43; 95% CI, 0.19, 0.99). These results continued to hold up when controlling for several cardiovascular risk factors and potentially confounding variables, supporting earlier observations that physical activity is beneficial in middle-age, and further suggesting that benefits may extend to the elderly male population as well.