Background and Methods To determine the associations between cigarette smoking, exercise capacity. and exercise heart rate response, data on 2323 men who had not undergone prior invasive cardiac procedures and who were referred for symptom-limited treadmill thallium studies were analyzed. Structured interviews yielded data on smoking habits. prior cardiac events, standard coronary risk factors, medication use. and other chronic diseases including lung disease. Aerobic impairment was defined as inability to achieve 85% of the age-predicted exercise capacity in metabolic equivalents (METs). Chronotropic incompetence was defined as failure to achieve 85% of the age-predicted maximum heart rate. Results: There were 385 regular smokers, 909 non-smokers, and 1029 ex-smokers. of whom 653 had quit more than 9 years before I’remote ex-smokers’). Aerobic impairment was present in 24% of non-smokers. 38% of smokers (Odds ratio [OR] = 1.98, 95% CI 1.53–2.56). and 27% of remote ex-smokers (OR = 1.18, 95% CI 0.941.48). These associations remained unchanged even after adjusting for age. thallium abnormalities, and clinical characteristics. Compared to nonsmokers. smokers had an age-adjusted exercise capacity of one MET less (P < 0.0001), irrespective of thallium abnormalities. Chronotropic incompetence was present in 10% of non-smokers, 21% of smokers (OR = 2.52, 95% CI 1.81–3.50), and 14% of remote ex-smokers lOR = 1.53,95% CI 1.12–2.09). These associations remained largely unchanged even after adjusting for age. thallium abnormalities, exercise capacity in METs. and clinical characteristics. Furthermore, age-adjusted heart rates were lower at all levels of sub-maximal exercise in smokers compared to non-smokers (P < 0.02 by ANOVA). In subset analyses of subjects who underwent coronary angiography similar independent associations of smoking with impaired aerobic capacity and chronotropic incompetence were noted, irrespective of the severity of coronary disease. Conclusions: Cigarette smoking is an independent predictor of impaired aerobic capacity and chronotropic incompetence. Smoking cessation may be associated with improvement of these exercise characteristics.