Sixteen patients with the Acquired Immunodeficiency Syndrome (AIDS) and cytomegalovirus retinitis were treated with ganciclovir alone (9 patients) or ganciclovir and zidovudine (6 patients). The duration of effective treatment, that is the number of weeks during which there was no deterioration in visual symptoms or retinal appearance, was comparable in both groups. However, six of the seven patients receiving concurrent therapy had to cease treatment temporarily because of bone marrow toxicity compared with one of the nine patients treated with ganciclovir alone. It is concluded that continuous concurrent therapy with oral zidovudine and intravenous ganciclovir is not possible unless unlimited supportive therapy including blood transfusion, is available.