Five cases of the acquired immune deficiency syndrome (AIDS), each exhibiting retinitis, were studied by DNA hybridization technique to detect viral infection. Five involved eyes were obtained at the time of autopsy from three patients who had received no treatment for the retinitis. Retinal biopsy specimens were obtained at the time of surgery from two other patients who developed retinal detachments after being treated with ganciclovir (dihydroxy propoxymethyl guanine). DNA hybridization revealed cytomegalovirus in retina and retinal pigment epithelium in all five specimens from patients who had not been treated with ganciclovir. No hybridization occurred in the two retinal biopsy specimens obtained from the ganciclovir-treated patients. These results suggest that in situ DNA hybridization is a highly specific and easily interpretable means of establishing the tissue diagnosis of viral retinitis.