In order to improve the determination of the causative agent in acute retinal necrosis syndrome, we evaluated the detection of intraocular antibody production to herpesviruses in 28 patients with this disease. Intraocular antibody production was determined by calculation of the Goldmann-Witmer coefficient whereby specific antibody titers in the inflamed eye and circulation are related to the total IgG content in ocular fluid and serum. Specific antibody titers to herpesviruses and Toxoplasma were determined by the indirect immunofluorescence technique. Thirty-five patients with ocular toxoplasmosis, cataract, or proliferative vitreoretinal disorders were tested as controls. By this technique, intraocular antibody production to varicella zoster virus or herpes simplex virus could be established in 16 (57%) of the patients with the typical clinical features of acute retinal necrosis, compared to none of the controls. Of the 33 affected eyes, 21 (64%) had a visual outcome of less than 20/200. We concluded that detection of intraocular antibody production to herpesviruses may be a useful diagnostic tool in establishing the causative agents in acute retinal necrosis.