Carbon monoxide is a ubiquitous air pollutant. It is found in cigarette smoke and emissions from motor vehicles, industrial processes, and poorly ventilated combustion sources. Dr. Penn and his colleagues at New York University Medical Center sought to determine whether chronic exposure to ambient levels of carbon monoxide is also a risk factor for developing atherosclerosis because this disease is the leading contributor to deaths by heart attack and stroke in the United States. The investigators had previously shown that the development of atherosclerotic plaques in cockerels (young roosters) was enhanced when the birds were exposed to cigarette smoke or treated with agents that stimulate cell proliferation, such as chemical carcinogens. In the study described here, Dr. Penn exposed cockerels to either air or defined levels of carbon monoxide for 16 weeks and measured the size of the atherosclerotic plaques in the abdominal aorta. He also examined whether carbon monoxide promoted atherosclerosis when the birds were fed a low cholesterol (0.1%) diet and exposed to carbon monoxide (100 ppm) at the same time. Finally, he determined whether carbon monoxide exposure altered the development of preexisting plaques.