The recent deaths of two workers with coronary artery disease (CAD) following exposure to carbon monoxide (CO) at work reinforced our appreciation of the hazard of this exposure to individuals with preexisting heart disease. Carbon monoxide acts to precipitate ischemia by reducing oxygen delivery to the myocardium. Animal and in vitro experiments suggest that CO may accelerate the development of atherosclerosis, particularly if exposure is in association with other risk factors. Thus, persons with known CAD who are exposed to CO at work are at risk for both the acceleration of the course of the underlying disease and for precipitation of acute ischemia or infarction following excessive exposure. Particular attention should be given to control of CO exposures in light of this hazard. For various reasons, preplacement evaluations or other job selection procedures do not adequately address his hazard. In view of the high prevalence of CAD in the U.S. and the high frequency of workplace exposure to CO, particular attention should be given to control of CO exposure through industrial hygiene measures.