The present invention relates to the use of a modified, oncolytic, herpes simplex virus (HSV) strain in the manufacture of a medicament for the oncolytic treatment of cancer, which HSV strain: (a) has a greater ability than HSV1 17+, HSV1 F and HSV1 KOS strains with equivalent modifications to replicate in or kill tumour cells; and (b) is obtainable by isolating an HSV strain from a host and modifying the isolated HSV strain to provide for selective replication in tumour cells. <;IMAGE>;