The correlation of the tropism of human immunodeficiency virus type 1 (HIV-1) isolates for MT-2 cells with response to zidovudine and didanosine treatment and with development of drug resistance was studied. Patients with MT-2-negative but not MT-2-positive HIV-1 had a significant increase in CD4+ lymphocyte counts during the first 6 months of treatment. In both groups and for both drugs, the rate of CD4+ lymphocyte decline decreased after the start of treatment. MT-2-positive isolates were more likely than MT-2-negative isolates to show reduced sensitivity to zidovudine and didanosine. Because the differences in zidovudine sensitivity were first evident after 12 months of treatment, drug resistance was probably not the cause of poor response early in zidovudine treatment in patients with MT-2-positive HIV-1. Thus, patients with MT-2-positive virus have limited benefit from treatment with single nucleoside analogues. Knowledge of MT-2 cell tropism may be important in clinical trials and for choosing treatments for patients.