A possible relationship between passive smoking and coronary heart disease has been widely debated during the past decade. Convincing evidence links environmental (passive) tobacco smoke exposure to heart disease morbidity as well as mortality. In the United States, 37,000 coronary heart disease deaths per year are attributed to environmental tobacco smoke exposure, accounting for 70% of all deaths caused by environmental tobacco smoke. The analysis of 10 epidemiologic studies indicated a consistent dose-response effect related to exposure, but more proof is still needed. Evidence indicates that nonsmokers are more sensitive to smoke, including cardiovascular effects, and that sidestream smoke contains higher concentrations of gas constituents, including carbon monoxide. Pathophysiological and biochemical data after short- and long-term environmental tobacco smoke exposure show changes in endothelial and platelet function as well as exercise capacity similar to those in active smoking. Therefore, passive smoking is a relevant risk factor for heart disease morbidity and mortality.