Inducible nitric oxide synthase (iNOS) is a high-output isoform of NOS that produces nitric oxide (NO), a nonspecific immune effector molecule. In some animal models of autoimmunity, the induction of iNOS has been shown to lead to inflammation and tissue damage, and it has been suggested that iNOS is an immune mediator in humans as well. Using in situ hybridization and immunohistochemical techniques, we demonstrate that iNOS mRNA and protein are present in the coronary arteries of transplanted human hearts with accelerated graft arteriosclerosis (AGA). iNOS is expressed in cells morphologically consistent with macrophages in the neointima of 7 of 10 of the transplanted vessels with AGA that were examined. In serial sections, these same cells express the macrophage marker CD68. In contrast, iNOS is absent from five native coronary arteries with atherosclerosis and absent from two normal coronary arteries. Although iNOS is expressed in macrophages in AGA, its role in the pathogenesis of AGA is unknown.