The dideoxynucleosides 3'-azido-3'-deoxythymidine (zidovudine, AZT) and 2',3'-dideoxycytidine (ddC) are potent inhibitors of human immunodeficiency virus (HIV) in vitro and improve surrogate measures of HIV infection in vivo. Long-term administration of AZT has been associated with significant hematologic toxicity and has resulted in virus with reduced in vitro susceptibility. Although ddC does not have significant hematologic toxicity and has not shown cross-resistance with AZT in vitro, ddC has been associated with a severe dose-related peripheral neuropathy. Given the independent activity of each agent, their non-overlapping toxicity profiles, and the absence of cross-resistance, it was hypothesized that concurrent administration of AZT and ddC in low doses might be at least as active as either drug given individually at higher doses in the treatment of HIV infection. It was further hypothesized that combined low doses of both drugs would reduce the incidence of serious side effects and the development of resistance. A phase I/II dose-finding trial of six different regimens of combination AZT and ddC in persons with acquired immunodeficiency syndrome (AIDS) or advanced AIDS-related complex was initiated to test these hypotheses.