The inducible isoform of nitric oxide synthase has been detected in cytomegalovirus (CMV)-infected retinas from acquired immunodeficiency syndrome (AIDS) patients by immunohistochemistry and NADPH-diaphorase staining. Subsequent immunohistochemistry using antibodies against CMV antigens and glial fibrillary acidic protein (GFAP) demonstrated that inducible NOS was localized in CMV-infected glial cells, particularly Müller cells. These findings indicate that inducible NOS is expressed in vivo in the human retina as a result of viral infection, and suggest that high levels of NO production might be involved in CMV-induced retinitis.