Objective:To investigate the correlation between the levels of the spontaneous apoptosis index(SAI),expressions of P53 and the radiosensitivity of rectal carcinoma.Methods:Thirty-six patients with rectal carcinoma who received preoperative radiotherapy were registered in this study.Specimens were obtained by biopsy before irradiation and after operation.The apoptotic index(AI) of each tissue specimen was evaluated by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphophate nick-labling (TUNEL) assay and expressed as a percentage,Immunohistochemical evaluation for P53 and the radiosensitivity of the tumor was also peformed.Results:Consistent low levels of spontaneous apoptosis were observed in all specimens evaluated,the mean AI of spontaneous apoptosis was 0.98%+/-0.31%;There was an inverse correlation between SAI and the expression of P53 protein(r=0.72,P0.98% was significantly higher than that of SAI