In this paper, two possible cases of acute carbon monoxide poisoning previously not identified in the medical and historical literature are discussed. The first concerns the famous Byzantine Emperor Julian the Apostate, who may have suffered mild carbon monoxide poisoning from which he quickly and completely recovered. The second case involves his successor, Jovian, who may have succumbed to severe carbon monoxide poisoning. Both cases were in all likelihood due to the burning of coal in braziers, a usual method of indoor heating during that epoch.