Reports had shown that between 24 and 78 percent of patients with Acquired Immune Deficiency Syndrome treated with zidovudine for four to six months developed significant bone marrow toxicity, requiring zidovudine dosage alterations, blood transfusions, or cessation of therapy. While zidovudine efficacy had been demonstrated in patients with AIDS or ARC, it was also evaluated in patients with AIDS-related illness, as well as in asymptomatic HIV-infected individuals. The clinically significant zidovudine-induced anemia developing in a substantial number of these patient groups and requiring blood transfusions could cause a significant drain on the blood supply in the United States.