PURPOSE: Preoperative chemoradiotherapy (CRT) is the standard treatment option in locally advanced rectal cancer. The tumor response is assessed through the tumor and nodal downstaging and the tumor regression grade. Currently, there is no method to predict tumor response to CRT. We aimed to evaluate whether p21 and p53 expressions could be a reliable predictors of pathological response to CRT. METHODS: Fifty patients with locally advanced rectal cancer were treated with preoperative radiotherapy combined with mitomycin C and capecitabine. p21 and p53 immunohistochemical staining were performed on pretreatment biopsies and the results were compared with tumor regression according to grading systems by Dworak (TRG grades) and by Wheeler (RCRG grades). RESULTS: Testing RCRG grades in relation to p21 expression showed statistically significant difference (p=0.021). RCRG 3 (poor response) was more frequent in the group of patients with low p21. According to Dworak, grade 4 (complete regression) was more frequent in the group of patients with positive p21 expression (p=0.032). Significant difference in p21 expression in grade 4 group compared with all other grade groups was also found (p=0.007). Patients with immune expression of p21 had significantly higher percentage of complete regression in comparison to the patients with low expression of p21. We haven't found any correlation between p53 expression and histopathological (HP) as well as regression grades. CONCLUSION: According to both grading systems, our results suggest that p53 expression does not, but p21 expression does predict pathological response to preoperative CRT.