A first-in-human trial indicates that treatment with an oncolytic virus is associated with increased survival in people with glioblastoma. In the phase I trial, 41 people with recurrent glioblastoma received intra-tumoural injection of an oncolytic herpes simplex virus (HSV) known as CAN-3110. The treatment produced an inflammatory tumour microenvironment that was favourable for immune responses against the tumour and was associated with increased survival time, particularly among people with HSV1 seropositivity.