Cumulative evidence suggests that the induction of the antioxidant/anti-inflammatory heme oxygenase (HO)-1 may play a protective role in allergic inflammation. HO-1 suppresses mast cell degranulation and cytokine synthesis. The up-regulation of the HO-1 pathway has a significant protective effect against airway inflammation, mucus hyper-secretion, and hyper-responsiveness in a model of allergic asthma. Moreover, HO-1 inhibits T cell-dependent skin inflammation and differentiation and function of antigen-presenting cells. The precise underlying mechanisms for HO-1-based protection against allergic inflammation are not yet completely understood, but appear to involve the protective effects of HO-1 by-products, such as carbon monoxide (CO), biliverdin/bilirubin and free iron. Among the HO-1 by-products, CO has been shown to mimic some protective actions of HO-1 in allergic inflammations. This article reviews the latest knowledge, recent patent and studies on the protective roles and mechanisms of HO-1/CO in inflammation and allergy.