Novel D-2'-azido-2',3'-dideoxyarabinofuranosyl-4'-thiopyrimidines and purines have been synthesized, starting from L-xylose via azidation at the 2'-position as a key step. Most of the final nucleosides exhibited toxicity-dependent anti-HIV-1 activity, among which D-alpha-adenine analogue was found to be the most cytotoxic.