We delivered preoperative radiotherapy with intratumoral injection (ITI) of radiosensitizer, 5-bromodeoxyuridine for patients with locally advanced cancer originating in lower rectum. We identified immunohistochemically the tumor cells that incorporated BrdU, and evaluated the radiosensitizing effect of BrdU histopathologically. (1) BrdU-labeled nuclei of cancer cells were identified in twenty-three cases out of 24 (96%). Mean labeling index was 18.5%, and tended to decrease as time progressed. (2) The parameters representing volume of residual tumor nests (Tumor Nest Index) or degrees of tumor extension (Depth of Extramural Tumor and Depth of Extramural Fibrosis) for patients with ITI were compared to those for patients without ITI. Those parameters tended to decrease when ITI was combined with radiotherapy. However, the difference was not significant statistically. Intratumorally injected BrdU can incorporate with cancer cells, but a methodological invention is required for yielding clinically measurable radiosensitization.