The herpes simplex virus (HSV) ICP34. 5 null mutant 1716 replicates selectively in actively dividing cells and has been proposed as a potential treatment for cancer, particularly brain tumours. We present a clinical study to evaluate the safety of 1716 in patients with relapsed malignant glioma. Following intratumoural inoculation of doses up to l0 5 pfu, there was no induction of encephalitis, no adverse clinical symptoms, and no reactivation of latent HSV. Of nine patients treated, four are currently alive and well 14–24 months after 1716 administration. This study demonstrates the feasibility of using replication-competent HSV in human therapy.