1-(2-Azido-2,3-dideoxy-beta-D-erythro-pentofuranosyl) thymine (2'-NN3ddThd) was synthesized from 1-(5-O-trityl-2,3-anhydro-beta-D-lyxofuranosyl) thymine by two different procedures. Method A prepared the title compound by opening of the oxirane ring with LiEt3BH followed by mesylation of the 2'-hydroxyl function, introduction of the 2'-azido substituent and deblocking of the 5'-function. In method B nucleophilic opening of 3'-deoxy-5'-O-(tert-butyldimethylsilyl)-5-methyl-2,2'-anhydrouridine was carried out with sodium azide in hexamethylphosphoramide in the presence of benzoic acid. Single X-ray crystallographic studies indicated a solid state conformation (3T2), which was opposite to that of the A form of AZT (2T3) but closely resemble that of 1-(2-fluoro-2,3-dideoxy-beta-D-ery thropentofuranosyl)thymine (2'FddThd) (3T2) and of 3'-azido-2',3'-dideoxy-2,6-diaminopurine riboside (3'-N3ddDAP) (3T2). Whereas the latter displayed significant inhibitory activity against human immunodeficiency virus (HIV) replication, 2'-FddThd and 2'-N3ddThd were essentially inactive.