Objectives. We investigated associations of smoking and coronary heart disease (CHD) by age. Methods. Data came from the Pooling Project on Diet and Coronary Heart Disease (8 prospective studies, 1974-1996; n = 192 067 women and 74 720 men, aged 40-89 years). Results. During follow-up, 4326 cases of CHD were reported. Relative to never smokers, CHD risk among current smokers was highest in the youngest and lowest in the oldest participants. For example, among women aged 40 to 49 years the hazard ratio was 8.5 (95% confidence interval [CI] = 5.0, 14) and 3.1 (95% CI = 2.0, 4.9) among those aged 70 years or older. The largest absolute risk differences between current smokers and never smokers were observed among the oldest participants. Finally, the majority of CHD cases among smokers were attributable to smoking. For example, attributable proportions of CHD by age group were 88% (40-49 years), 81% (50-59 years), 71% for (60-69 years), and 68% (70+ years) among women who smoked. Conclusions. Among smokers, the majority of CHD cases are attributable to smoking in all age groups. Smoking prevention is important, irrespective of age. (Am J Public Health. Published online ahead of print June 13, 2013: e1-e7. doi:10.2105/AJPH.2012.301091).