Tissue for pathologic examination was obtained from three cases of acute retinal necrosis syndrome. Virus particles belonging to the herpesvirus family were demonstrated in retinal biopsies from two patients, one of whom was immunosuppressed. Despite removal of large biopsy specimens, the retina has remained attached for 20 months postoperatively in one case and for three months in the other. In a third patient with acquired immune deficiency syndrome, the clinical course and postmortem immunopathology were suggestive of a herpes simplex virus infection, initially affecting the retina and subsequently the optic nerves, chiasm, tracts, and central nervous system. These cases illustrate that the virus associated with the acute retinal necrosis syndrome is easily demonstrable using vitrectomy and endoretinal biopsy in the acute phase of the disease, but may be difficult to demonstrate in chronically detached atrophic retinas.