4′-C-Ethynyl-β-D-arabino- and 4′-C-ethynyl-2′p-deoxy-β-D-ribo-pentofuranosylpyrimidine and -purine nucleosides were synthesized and evaluated for their in vitro anti-HIV activity. The key intermediate, 4-C-ethynyl- or 4-C-triethylsilylethynyl-D-ribo-pentofuranose, was prepared from D-glucose and glycosidated with various pyrimidine or purine bases. The arabino pyrimidine derivatives were prepared from the corresponding ribo derivatives via O2,2′-anhydro nucleosides. The 2′-deoxy-ribo derivatives were synthesized by radical reduction of 2′-bromo or 2′- phenoxythiocarbonyloxy nucleosides. Among these 4′-C-ethynyl nucleosides, seven analogues proved to be potent against HIV-1 in vitro with EC50 values ranging from 0.0003 to 0.03 μM. These compounds also exerted activity against clinical and multi-dideoxy-nucleoside-resistant HIV-1 strains with comparable EC50 values. Three such 4′-C-ethynyl-2′-deoxypurine analogues including 4′-C-ethynyl-2′-deoxyadenosine and 4′-C-ethynyl-2,6-diamino-2′-deoxy-purine were less cytotoxic [selectivity indices (SIs): 975-2733] than three 4′-C-ethynyl-2′-deoxycytidine analogues (SIs: 63-363). 4′-C-Ethynyl-5-fluoro-2′-deoxycytidine was least toxic (SI: >3333) and potent against all HIV strains tested.