AMENDED: AZT will be administered orally every 4 or 5 hours. Patients in the second arm discontinue AZT and take ddC as two tablets every 8 hours. Duration of the study is 1 year with interim analysis done at 6 months after 75 percent enrollment and at end of the study. Original design: Patients with AIDS or advanced ARC who have been receiving at least 500 mg/day of AZT for at least 48 weeks are randomized to 1 of 2 treatment arms. Patients in the first treatment arm continue their current dose of AZT.