Zidovudine (AZT, 3'-azido-3'-deoxythymidine) was shown in a controlled trial to decrease the incidence of mortality, reduce the frequency of opportunistic infections, and provide other clinical benefits to patients with acquired immune deficiency syndrome (AIDS) and advanced AIDS-related complex. Two hundred twenty-nine patients who participated in the double-blind placebo-controlled trial of zidovudine were enrolled in an open-label study. As of August 31, 1987, and interim analysis indicated that patients receiving zidovudine continued to derive benefits from therapy. Survival rates of zidovudine-treated patients were higher than those that might have been expected from previous experience with similar patients. Patients continued to experience episodes of opportunistic infections; however, these infections were either of decreased severity or were more responsive to conventional therapy. The increase in median CD4 cell counts that occurred initially was followed by a gradual decline to near baseline values. Hematologic toxicities continued to be the major laboratory abnormality associated with drug administration; however, new or more frequent toxicity was not observed with more prolonged therapy. Progressive bone marrow suppression did not appear to be associated with prolonged administration. Overall, patients originally enrolled in the double-blind trial continued to receive clinical benefit from zidovudine therapy.