After baseline screening, patients are randomized to one of two treatment arms (ddI or ddC). Subjects are evaluated biweekly for the first 4 weeks of study, at 2 months, and every other month thereafter. Three dose levels of ddI (based on patient's weight at study entry) are compared with two dose levels of ddC (also based on patient weight). Patients who reach a new progression-of-disease primary endpoint after at least 12 weeks of treatment or a drug intolerance endpoint have the option of switching over to the alternate study drug; however, participants are encouraged to remain on their original drug assignment whenever possible. For any switchover, patients must be off the originally assigned drug for at least 72 hours before switching. Only one switchover is allowed.