The cellular content of heme, derived from the breakdown of heme proteins, is regulated via the heme oxygenase (HO) enzyme system. HO catalyzes the rate-limiting step in heme degradation resulting in the formation of iron, carbon monoxide, and biliverdin. Recent studies have focused on the biologic effects of product(s) of this reaction, which have important antioxidant, antiapoptotic, anti-inflammatory, and cytoprotective properties. Two isoforms of the HO enzyme have been described: an inducible isoform (HO-1) and a constitutively expressed isoform (HO-2). Induction of HO-1 occurs as a beneficial response to several injurious stimuli and has been implicated in many clinically relevant disease states including sepsis, hypertension, atherosclerosis, and acute lung and kidney injury. This review focuses on the role of HO-1 in kidney diseases. © Current Medicine Group LLC 2009.