On the basis of observation that acyclovir potentiates the in-vitro antiviral activity of 3-azido-2',3'-dideoxythymidine (also known as azidothymidine or zidovudine) against human immunodeficiency virus (HIV), we administered a regimen of azidothymidine and acyclovir to eight patients with the acquired immunodeficiency syndrome (AIDS) or AIDS-related complex. An oral regimen of 100 mg of azidothymidine and 800 mg of acyclovir every 4 hours was in general well tolerated, with the principal toxicity being megaloblastic erythroid changes. The pharmacokinetics of the two drugs were independent of each other. Six patients received the drug combination for at least 10 weeks; all had increased numbers of T4+ lymphocytes (P = 0.028), and two of three assessable patients had reversal of anergy. Two patients tested positive for serum HIV p24 antigen at entry, but became negative with treatment. Data for this small group suggest that this drug combination can be tolerated in patients with severe HIV infections; this study can be used as a basis for larger studies of this drug combination.