Patients must have active CMV retinitis in one or both eyes, despite prior systemic therapy. Following medical evaluation, the decision is made whether to treat the eye(s) immediately or to watch the eye(s) carefully for advancement of the retinitis. Eyes with sight-threatening lesions or eyes without functional vision are treated immediately and eyes without sight-threatening lesions are randomly chosen for either immediate or deferred therapy. DHPG is given by injection with a very fine needle twice a week for the first 3 weeks and once a week for the remaining 24 weeks.