beta-L-2',3'-Dideoxycytidine (beta-L-ddC) and beta-L-5-fluoro-2',3'-dideoxycytidine (5-F-beta-L-ddC) were prepared and shown to have potent activity against human immunodeficiency virus type 1 (HIV-1) and hepatitis B virus (HBV). These compounds were compared with beta-D-2',3'-dideoxycytidine (beta-D-ddC) and two beta-L-oxathiolane nucleosides (beta-L-3'-thio-2',3'-dideoxycytidine and beta-L-5-fluoro-3'-thio-2',3'-dideoxycytidine) in terms of anti-HIV and anti-HBV activity, cytotoxicity, and development of HIV-1 resistance. Compared with beta-D-ddC, the beta-L-dideoxycytidine nucleosides had similar anti-HIV-1 activities, significantly greater anti-HBV activities, and decreased toxicities to a B-cell line, T-cell lines, and human bone marrow progenitor cells. HIV-1 strains resistant to beta-D-ddC were susceptible to the beta-L-ddC analogs. Compared with the beta-L-oxathiolane nucleosides, beta-L-ddC and 5-F-beta-L-ddC had similar anti-HIV-1 activities, decreased anti-HBV activities, and greater toxicities to B- and T-cell lines and bone marrow progenitor cells. There were similarities between the beta-L-ddC and beta-L-oxathiolane nucleosides in the rate of development and pattern of resistant HIV-1 selection. While the in vitro activity and cytotoxicity profiles of the beta-L-ddC nucleosides differed from those of the beta-D-ddC and beta-L-oxathiolane nucleosides, the data presented herein suggest that the sugar configuration of a dideoxynucleoside analog may play a major role in the rate of development and the pattern of HIV-1 resistance.