Primary: To determine the clinical effect of zidovudine (AZT) compared to placebo in terms of time to progression to AIDS (i.e., occurrence of major opportunistic infections, dementia, and malignancies) or death. Initial drug assignment will be changed to open-label AZT for patients who experience a sustained decline in CD4 lymphocyte concentration to less than 200 cells/mm3, but analysis will be based on initial treatment assignment. To determine the immunologic effect of AZT compared to placebo in terms of time to drop in CD4 lymphocyte concentration of at least 25 percent from baseline assessment. To determine the antiviral effect of AZT compared to placebo in eradicating or suppressing HIV. Secondary: To determine the effect of AZT compared to placebo on the immune status of HIV-infected patients by comparing lymphocyte profiles and indices. To determine the long-term toxicities of AZT compared to placebo in terms of abnormalities in BL, hepatic function, renal function, skin, gastrointestinal system, and central nervous system. To describe the natural history of AIDS related complex (ARC) in placebo patients in terms of initial CD4 lymphocyte concentration and the Walter Reed staging system.