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To evaluate the safety of zalcitabine (dideoxycytidine; ddC) alone and in combination with zidovudine (AZT) versus AZT alone when administered to asymptomatic patients with a CD4 count = or < 200 cells/mm3 and symptomatic patients with a CD4 count = or < 300 cells/mm3. To compare the effectiveness of ddC alone and in combination with AZT versus AZT alone. ddC has been shown to demonstrate an antiviral effect. AZT has been shown to significantly decrease mortality and reduce the frequency of opportunistic infections in patients with AIDS or advanced ARC. After 1 year of AZT therapy, the effectiveness tends to diminish and patients progress with more opportunistic infections and higher mortality rates. Because of the demonstrated antiviral activity, absence of hematologic toxicity, and lack of cross tolerance in laboratory studies of ddC, a study to investigate the long-term effectiveness of ddC in patients with HIV infection who have received AZT therapy is warranted. ddC has been shown to demonstrate an antiviral effect. AZT has been shown to significantly decrease mortality and reduce the frequency of opportunistic infections in patients with AIDS or advanced ARC. After 1 year of AZT therapy, the effectiveness tends to diminish and patients progress with more opportunistic infections and higher mortality rates. Because of the demonstrated antiviral activity, absence of hematologic toxicity, and lack of cross tolerance in laboratory studies of ddC, a study to investigate the long-term effectiveness of ddC in patients with HIV infection who have received AZT therapy is warranted. Patients are randomly assigned to 1 of 3 treatment groups. In study arm 1, patients receive AZT plus ddC placebo. In study arm 2, patients receive ddC plus AZT placebo capsules. In study arm 3, patients receive ddC plus AZT. Patients are seen every other week for first 8 weeks and monthly thereafter. Patients are stratified by HIV disease status, length of time receiving AZT, and systemic or local Pneumocystis carinii pneumonia (PCP) prophylaxis. Patients who reach a clinical AIDS-defining endpoint are offered open-label combination therapy.
Inclusion Criteria Concurrent Medication: Required: * Zidovudine (AZT) = or \> 300 mg/day for 6 weeks prior to study entry. Allowed: * Chemoprophylaxis for Pneumocystis carinii pneumonia (PCP), candidiasis, and herpes. * 21 day course of adjuvant systemic corticosteroids for moderate to severe PCP. * Maintenance treatment with pyrimethamine, sulfadiazine, amphotericin, fluconazole, ketoconazole, acyclovir, ganciclovir, or medications for tuberculosis or Mycobacterium avium for patients who have recovered from toxoplasmosis, cryptococcosis, candidiasis, herpes virus infections, cytomegalovirus infections, tuberculosis or Mycobacterium avium intracellulare. * 14 day course of metronidazole. * Erythropoietin and megace if clinically indicated. * Isoniazid if patient has no peripheral neuropathy at entry and is taking pyridoxine = or \> 50 mg/day concomitantly. * Phenytoin if patient has \< grade 2 peripheral neuropathy at entry and has been stable on phenytoin = or \> 3 months. Patients must have: * Ability and willingness to give informed consent. * Written informed consent from a parent or guardian if \< 18 years old. * Been tolerating zidovudine (AZT) therapy. * Diagnosis of HIV infection. Exclusion Criteria Co-existing Condition: Patients with the following conditions or symptoms are excluded: * Kaposi's sarcoma or other malignancy requiring therapy. * Active opportunistic infections. * Peripheral neuropathy as manifested by complaints of moderate pain, burning, numbness, or tingling in hands/arms or feet/legs; moderate sensory deficit in the upper or lower extremities; or motor weakness in the upper or lower extremities. Concurrent Medication: Excluded: * Other experimental medications. * Other anti-HIV drugs. * Biologic response modifiers. * Cytotoxic chemotherapy. * Drugs that could cause peripheral neuropathy including phenytoin not specifically allowed, hydralazine, nitrofurantoin, vincristine, cisplatinum, dapsone, disulfiram, and diethyldithiocarbamate. Concurrent Treatment: Excluded: * Radiation therapy. Patients with the following are excluded: * Active opportunistic infection. Must have ended acute therapy at least 14 days prior to study entry. * Peripheral neuropathy = or \> grade 2. * History of intolerance to 500 to 600 mg/day of zidovudine (AZT) as manifested by the same recurrent grade 3 toxicity requiring dose interruptions and dose reductions to \< 500 mg/day or any prior grade 4 toxicity. * Prior development of peripheral neuropathy on ddI = or \> grade 2. Prior Medication: Excluded: * Dideoxycytidine (ddC). Required: * Zidovudine (AZT) for total of at least 24 weeks; and included within that time period, AZT = or \> 300 mg/day for 6 weeks prior to the study entry.