On the basis of current knowledge, chronic exposure to carbon monoxide could influence the onset or severity of a myocardial infarction by two main mechanisms. (1) Animal experiments have indicated that the atheromatous process can be accelerated by sustained high carboxyhemoglobin (COHb) levels. (2) Once a coronary occlusion occurs, the severity of ischemia could be increased by the presence of carbon monoxide even in relatively low concentrations. There is very little evidence in man supporting these hypotheses. This research was undertaken to investigate patients admitted to the Johns Hopkins Hospital Emergency Ward with symptoms of chest pain, to determine if carbon monoxide at admission might relate to the severity of the infarct. The possibility that chronic exposure might play a role in the severity was also evaluated. Previously Johns Hopkins issued a final report on the studies relating carboxyhemoglobin and sudden death in Baltimore, Maryland.