We have developed a novel replication-competent, oncolytic herpes simplex virus (HSV), named HF10, and have evaluated its anticancer efficacy in a variety of animal models. We report a pilot study of intratumoral injection of HF10 into subcutaneous nodules in patients with head and neck squamous cell carcinoma (HNSCC). HF10 efficiently infected human HNSCC cells and caused extensive tumor cell death without any significant adverse effects, suggesting that HF10 represents a promising therapy for HNSCC in humans. To assess the therapeutic potential of HF10 in human HNSCC, we performed a preliminary study of toxicity and efficacy in two patients with recurrent metastatic HNSCC. For each patient, a metastatic skin nodule was injected with HF10 once a day for 3 days. They were monitored for systemic adverse effects, and the injected nodules were excised at day 13 (patient 1) or day 15 (patient 2) after injection for histochemical examination. HF10 replicated, spread well in the tumor nodules, and caused cell death in a considerable population of tumor cells without any significant adverse effects.