An 83-year-old patient with advanced gastric cancer was given repeated endoscopic administration of OK-432 combined with systemic immunochemotherapy. The cancerous lesion gradually regressed following the treatment, and remarkable degeneration was observed in many tumor cells. Detailed immunohistochemical examination revealed that Leu 7-positive cells had markedly infiltrated the cancerous tissue. This finding suggests that NK cells may play an important role in tumor regression induced by intratumoral OK-432 therapy.