Heme oxygenase-1 (HO-1) is an inducible enzyme that catalyzes oxidative degradation of heme to form biliverdin, carbon monoxide (CO), and free iron. Biliverdin is subsequently reduced to bilirubin by the enzyme biliverdin reductase. Increasing evidence has indicated the critical role of HO-1 in cytoprotection and more diverse biological functions. Induction of HO-1 by various chemical inducers that are primarily cell stress inducers or by HO-1 gene transfection confers a protective capacity to cultured cells as well as to cells in several in vivo animal models. In addition, HO-1-deficient mice exhibit a significant increase in susceptibility to tissue injury. The cytoprotective action of HO-1 seems to be mainly a function of the antiapoptotic effects of the enzyme. HO-1 is believed to exert this antiapoptotic action by multiple mechanisms: (a) decreased intracellular pro-oxidant levels, (b) increased bilirubin levels, and (c) elevated CO production. CO may produce an antiapoptotic effect by inhibiting both expression of p53 and release of mitochondrial cytochrome c. HO-1 may also be a target in antitumor therapy because the growth of most tumors depends on HO-1. Our preliminary studies with an HO inhibitor showed a promising antitumor effect. This preliminary work warrants continued investigation for possible novel anticancer chemotherapy.