The present invention relates to an oncolytic virus which is, or is derived from, a clinical isolate which has been selected by comparing the abilities of a panel of three or more clinical isolates of the same viral species to kill tumor cells of two or more tumor cell lines in vitro and selecting a clinical isolate which is capable of killing cells of two or more tumor cell lines more rapidly and/or at a lower dose in vitro than one or more of the other clinical isolates in the panel.