Cytomegaloviral retinitis was diagnosed in nine eyes of seven patients with acquired immune deficiency syndrome (AIDS) on the basis of the characteristic ocular findings and a positive culture for cytomegalovirus (CMV) obtained systemically. Treatment with ganciclovir was begun on a protocol which provided two weeks of inpatient therapy at daily doses of 2.5 to 10 mg/kg followed by outpatient therapy at a reduced dosage three to six days per week. Outpatient maintenance dosage ranged from 15 mg/kg per week to 30 mg/kg per week. In seven eyes of six patients the treatment decreased retinal inflammation and stabilised the margins of the lesions. Six patients have tolerated long term maintenance therapy for 10-30 weeks. Six of seven patients (85%) in this study developed side effects from ganciclovir which required periods of a reduction in dosage or interruption of therapy. The side effects from ganciclovir included neutropenia, thrombocytopenia, drug fever, and neuropathy. Physicians using ganciclovir in AIDS patients must watch for the signs of drug toxicity and adjust treatment accordingly. Ganciclovir appears to be a promising therapy for CMV retinitis, but further work is necessary to determine the best regimen for optimal efficacy with minimal side effects.