Patients on amphotericin, pyrimethamine, sulfadiazine, trimethoprim/sulfamethoxazole, ganciclovir, intravenous pentamidine, intravenous acyclovir or oral acyclovir or other bone marrow or renal toxic drugs may not tolerate concomitant ddC. If these drugs are given concomitantly with ddC, patients should have frequent clinical and laboratory assessments, as appropriate. Drugs that are nephrotoxic or have the potential to cause peripheral neuropathy might be expected to cause increased toxicity when co-administered with ddC. Drugs that could cause serious additive toxicity when co-administered with study medication will be allowed for treatment of an acute intercurrent illness or opportunistic infection at the discretion of the investigator. Their use may be allowed with interruption of study drug for up to 35 days per episode, for a total of 90 days for the study. If the patient's condition requires chronic administration of these medications, the patient will be discontinued from study medication and followed.