Patients accepted into the study are randomly assigned to receive either AZT or placebo. Treatment continues for a minimum of 104 weeks beyond the time the last patient enters the study. If the study medication causes toxic effects, the dose is decreased or temporarily stopped, and if the toxic effects are severe, then the medication will be stopped permanently. Participants visit the clinic every 2 weeks during the first 16 weeks and once a month thereafter. Throughout the study frequent blood samples are taken to monitor the effectiveness and safety of the treatment. AMENDED: The placebo arm has been discontinued as of August 3, 1989 and the AZT dose has been reduced.