Cytomegalovirus (CMV) retinitis presents with typical ophthalmologic appearance in patients with underlying immunosuppressive conditions. Fourteen patients with this disorder were diagnosed by culture of cytomegalovirus from urine or throat specimens, elevated complement fixation titers to cytomegalovirus, and characteristic funduscopic appearance. Ten of 11 had decreased CMV-specific cell-mediated immune responses. Three of seven who received no specific therapy improved after decreasing dosages of immunosuppressive drugs. Seven patients with progressive disease despite minimal immunosuppressive therapy were treated with adenine arabinoside at doses from 1 to 20 mg/kg of body weight per day. Daily dosages of 20 mg/kg . d in five patients were associated with decreased inflammatory activity and improvement of retinal lesions and quantitative decreases in urinary virus excretion. Adenine arabinoside administration was associated with significant gastrointestinal, hematologic, and neurologic side effects. Adenine arabinoside may have some beneficial effect on selected patients with progressive CMV retinitis.