The present invention relates to the use of nucleoside derivatives of formula I ##STR00001## wherein B signifies a 9-purinyl residue B1 of formula ##STR00002## or a 1-pyrimidyl residue B2 of formula ##STR00003## wherein the symbols are as defined in the specification, and of pharmaceutically acceptable salts thereof; for the treatment of diseases mediated by the Hepatitis C Virus (HCV), for the preparation of a medicament for such treatment and to pharmaceutical compositions containing such compounds.