The purpose of this study was to assess whether the understaging of rectal tumors seen after preoperative radiotherapy is due to the biological effect of the irradiation or reflect the difficulties of the pathologists in identifying lymph nodes from radiated specimens. To study this question a retrospective non randomized study was undertaken including 31 patients with rectal cancer and oncological resection, 16 after preoperative administration of 36 Gy and 15 without radiotherapy. Metastatic lymph nodes were found in 37% of patients of the radiated group and in 58% of no irradiated group, being de difference between them of 16% with a confidence interval from -15% to 40%. A mean of 13 and 18 lymph nodes were identified in the radiated and non radiated group respectively, being the difference between means of 5 with a confidence interval from -0.6 to 10. A 80% and 57% of the pathological reports in no radiated and radiated group respectively accomplished the criteria of reliability (> 13 lymph modes isolated), being the difference between proportions of 27% with a confidence interval from -3% to 57%. Based on the above stated results no conclusive answer can be addressed to the proposed question; both effects could participate in the downstaging. More studies including more patients are needed to give a reliable answer to the proposed question.