The observation that human immunodeficiency virus resistant to zidovudine can be isolated from patients with AIDS or AIDS-related complex during prolonged zidovudine treatment was made with a plaque reduction assay employing a CD4-expressing HeLa cell line. Fifty percent inhibitory concentrations of zidovudine for isolates from untreated patients ranged from 0.01 to 0.05 microM. In contrast, most isolates from these patients showed decreased sensitivity after 6 months or more of zidovudine administration. Isolates from several patients showed progressive, stepwise reductions in sensitivity. Zidovudine-resistant isolates exhibited cross resistance to 3'-azido-2',3'-dideoxyuridine and 3'-azido-2',3'-dideoxyguanosine but not to dideoxycytidine, dideoxyinosine, foscarnet (phosphonoformate), or several other compounds. The clinical implications of these findings have yet to be determine. Studies are in progress to assess sensitivity patterns of isolates from patients who are at earlier stages of human immunodeficiency virus disease or who are receiving other drug therapy and to characterize the mutations and mechanisms accounting for resistance.