Some recent papers on coronary artery heart disease, smoking, or carbon monoxide contain pieces of a provocative jigsaw puzzle. Can a coherent picture be assembled from the pieces? First, consider the excess mortality from coronary heart disease among cigarette smokers, first noted by epidemiologists looking at the smoking-lung-cancer problem. Coronary heart disease accounts for about half of all the excess deaths due to smoking (1). Hammond (2) showed in a prospective study of 441,000 men that deaths from coronary heart disease were more than three times greater in 45- to 54-year-old men smoking more than one half pack a day