Patients are randomized to receive oral AZT (at age-adjusted doses) or placebo. Patients are evaluated at weeks 2 and 4 and every 4 weeks thereafter; those who reach a study endpoint will have their treatment unblinded to allow the clinician to determine which treatment regimen the patient should then receive. Patients who meet the criteria for changes to open-label AZT will be given the appropriate age-adjusted dose without unblinding the original randomization assignment. Patients who complete or discontinue treatment are followed every 6 months for up to 2 years.