1-(3-Cyano-2,3-dideoxy-beta-D-erythro-pentofuranosyl)thymine (cyanothymidine) (3a) has been prepared by an unambiguous route starting from D-xylose. The relative and absolute stereochemistry of 3a and its anomeric isomer 9 have been confirmed by NOE experiments and by X-ray diffraction analysis. In antiviral tests vs HIV 3a was shown to be inactive, a surprising result in view of a preliminary disclosure claiming potent anti-HIV activity. The activity previously assigned to 3a is believed to be due to contamination of that sample with the known antiviral nucleoside analogue 5b.