Abstract: Pharmaceutical compositions which contain a xyloside or a lyxoside of a purine or pyrimidine base selected from the following compounds:1-(.beta.-D-xylofuranosyl)-cytosine, 1-(.beta.-D-xylofuranosyl)-thymine, 9-(.alpha.-D-xylofuranosyl)-adenine, 9-(.alpha.-D-xylofuranosyl)-guanine, 1-(.alpha.-D-xylofuranosyl)-cytosine, 1-(.alpha.-D-xylofuranosyl)-thymine, 1-(.beta.-D-lyxofuranosyl)-adenine, 9-(.beta.-D-lyxofuranosyl)-guanine, 1-(.beta.-D-lyxofuranosyl)-cytosine, 1-(.beta.-D-lyxofuranosyl)-thymine, 9-(.alpha.-D-lyxofuranosyl)-adenine, 1-(.alpha.-D-lyxofuranosyl)-guanine, 1-(.alpha.-D-lyxofuranosyl)-cytosine and 1-(.alpha.-D-lyxofuranosyl)-thymine, have been found to be useful in therapy for treating various viral diseases.