The double origin of carbon monoxide (CO) as an atmospheric pollutant or as an endogenous gaseous modulator of many pathophysiological processes prompted us to review some aspects of the bad side and of the good side of coin among the pleiotropic effects of CO. On the bad side of the coin, we focus on the interval form in acute CO poisoning, discussing experimental evidence suggesting that the delayed neuropathology after CO poisoning is a free radical-driven event. In this context, we challenge the mandatory place of hyperbaric oxygen therapy (HBO) in CO poisoning as a possible summation of oxy-radicals generated by HBO and the free radical cascade set in motion during the reoxygenation phase of acute CO-poisoning. We also discuss an opposing view, which provides evidence suggesting that HBO therapy actually decreases the load of free radicals in acute CO-poisoning and may be beneficial in preventing delayed neuropsychiatric sequelae.On the good side of the coin, we briefly outline the endogenous generation of CO and the leading role of heme-oxygenases (HO) in relation to the place of CO in biology and medicine. The main focus of this section is on the growing literature on CO and inflammation. Here we report on in-vitro and in-vivo studies on the modulation afforded by exogenously administered/endogenously produced CO in a variety of experimental and clinical settings of inflammation. Our recent studies on experimental models of allergic inflammation are also discussed, and the CO-releasing molecules envisaged as potential anti-inflammatory drugs suitable for clinical use.