Although much has been written on the subject of acute carbon monoxide poisoning, there is a dearth of literature on chronic carbon monoxide poisoning. This may be attributed to the fact that there is no unanimity of opinion as to whether frequent exposures to sublethal amounts of the gas over a considerable period of time causes any clinical phenoma or pathologic changes. Some authoritative textbooks on medicine either do not recognize the existence of chronic carbon monoxide poisoning as a clinical entity or lead one into doubt and pass the matter over with a mere reference to the subject. Because of this prevailing doubt, I have critically studied from the standpoint of symptomatology and clinical manifestations a series of ninety-seven patients who were repeatedly subjected to sublethal doses of carbon monoxide, at varying intervals over prolonged periods. The results of these studies, as well as those which I1previously