Nuclear factor-kappaB (NF-κB), especially p65 subunit, has been associated with origin and progression of cancer as well as with the resistance to radiotherapy and chemotherapy in experimental models. The aim of the present study was to determine expression of NF-κB/p65 in tumor specimens before and after treatment of rectal cancer patients and to evaluate possible relationship between expression of NF-κB/p65 before and after (chemo)radiotherapy, other tumor characteristics and the clinical outcome. Furthermore, NF-κB/p65 was studied in relationship to pathologic response to preoperative (chemo)radiotherapy. Fifty patients with rectal cancer undergoing neoadjuvant (chemo)radiotherapy and surgery were included in the study. Pre-treatment rectal cancer specimens were obtained from diagnostic colonoscopy. Post-treatment rectal cancer specimens were obtained from surgically removed part of the rectum with the tumor. NF-κB/p65 expression was determined by immunohistochemistry and analysis was performed both in biopsies and in post-treatment tumor samples. Cytoplasmic positivity in tumor cells and nuclear positivity in lymphocytes were detected. High NF-κB/p65 positivity in pre-treatment tumor samples was significantly associated with shortened overall survival (OS). Disease-free survival (DFS) tends to be shortened as well. In post-treatment tumor samples, high NF-κB/p65 positivity was neither associated with shortened OS nor with shortened DFS. In post-treatment samples residual tumor cells deeply infiltrating the wall of the rectum with high NF-κB/p65 expression were found. The cells were linked to significantly worse clinical outcome in terms of shortened OS and DFS. NF-κB/p65 positivity did not correlate with pathologic response to preoperative (chemo)radiotherapy. In conclusion, our data suggest that high level of NF-κB/p65 subunit may be associated with more aggressive features of the tumor, higher metastatic potential, and shortened overall survival, but it does not correlate with resistance to (chemo)radiotherapy. Consequently, the level of NF-κB/p65 may help to select those patients who have poor prognosis and are candidates for more intensive anticancer therapy. For these purposes both pre-treatment and post-treatment tumor samples may be used.