In order to evaluate host immune response to cancer, many methods have been applied. However, in the field of oral squamous cell carcinoma, evaluation of host immune response on the basis of proliferative activity of tumor-infiltrating T-cells (TIL) has not been reported. Therefore, we applied double immunohistochemical staining of proliferation markers Ki-67 and CD8 to surgically resected and paraffin-embedded tissue sections for 35 cases of oral squamous cell carcinoma. With this method, there was a significant correlation between the percentage of Ki-67+CD8+TIL and the intra-tumor epithelium infiltration rate of CD8+TIL (P=0.0237). In the process of analysis, we found that the proliferative activity of CD8+TIL tended to correlate (P=0.0859) with clinical N factor (lymph node metastasis), which was previously reported to suppress host immune response. We therefore assumed there was another factor inducing host immune response. The proliferative activity of CD8+TIL was well correlated with preoperative radiotherapy (P=0.0200) while there was no significant correlation between the proliferative activity of CD8+TIL and other clinical factors; age, tumor size, clinical stage, pathological N factor (P=0.5410, 0.7769, 0.1041, and 0.1072, respectively). Our present results strongly imply that preoperative radiotherapy is a very important factor in oral squamous cell carcinoma inducing host immune response regardless of the clinical factors present.