In a 6-week clinical trial 4 dose regimens of 3'-azido-3'-deoxythymidine (AZT), a thymidine analogue with potent anti-viral activity against HTLV-III in vitro, were examined in 19 patients with the acquired immunodeficiency syndrome (AIDS) or AIDS-related complex (ARC). AZT was given intravenously for 2 weeks, then orally for 4 weeks at twice the intravenous dose. AZT was well absorbed from the gut and crossed the blood-brain barrier. Therapeutic levels were maintained with 5 mg given intravenously or 10 mg given orally every 4 h. Treatment was not limited by side-effects, the commonest of which were headaches and depression of white-cell counts. 15 of the 19 patients had increases in their numbers of circulating helper-inducer T lymphocytes (p less than 0.001) during therapy, 6 who were anergic at entry showed positive delayed type hypersensitivity skin test reactions during treatment, 2 had clearance of chronic fungal nailbed infections without specific anti-fungal therapy, 6 had other evidence of clinical improvement, and the group as a whole had a weight gain of 2.2 kg. Also, with the highest dose regimen cultures of peripheral blood mononuclear cells for HTLV III became negative.