Radiotherapy (RT) is one of the most important treatments for prostate cancer. Although RT can kill cancer cells through direct and indirect effects of radiation, it occasionally induces an abscopal effect whereby localized radiation treatment is associated with elimination of metastatic cancer at a distance from the irradiated area. Thus, RT may induce an effective antitumor immune response, although the mechanism involved has remained unclear. The present was designed to evaluate this effect of RT in 36 patients with prostate cancer who provided informed consent prior to enrollment in this clinical trial. Peripheral blood samples were collected periodically after low-dose-rate (LDR) prostate brachytherapy, and lymphocyte subsets were analyzed by flow cytometry. The proportion of activated T cells (CD3+HLA-DR+, CD4+HLA-DR+ and CD8+HLA-DR+) in peripheral blood revealed a gradual and bimodal increase after LDR brachytherapy, whereas memory CD8+ T cells bimodally decreased after treatment. The ratios of activated T cells and regulatory T cells gradually increased after the treatment. Thus, LDR brachytherapy was demonstrated to induce effective immune responses in patients. This increase of activated T cells may contribute to maintenance of remission and reduction of relapse rates.