The efficacy and toxicity of ganciclovir given by intravenous or intravenous plus intravitreal injection were studied in nine patients with cytomegalovirus (CMV) retinitis; seven with AIDS and two with drug induced immunodeficiency. Five patients had retinitis with macular involvement in six sighted eyes; six patients had only peripheral retinitis in seven eyes. In two patients (two eyes) with macular involvement intravenous plus intravitreal injection of ganciclovir preserved sight; intravenous infusion alone did not in four eyes of three other patients. In seven eyes (six patients) with peripheral retinitis vision was retained regardless of the route of ganciclovir treatment. Following intravenous ganciclovir drug levels in the vitreous fluid were 1.4-2.2 mmol/l, that is, 44 and 65% of the concomitant serum concentration. Clinically and at necropsy three eyes showed no evidence of toxicity from intravitreal injection of ganciclovir. All of five patients with AIDS who received intravenous ganciclovir for more than one week developed leucopenia. CMV retinitis of the macula may be benefited with minimal drug toxicity by intravitreal injection of ganciclovir. Treatment of peripheral CMV retinitis in patients with AIDS may be optional.