OBJECTIVE Preoperative adjuvant radiation combined with chemotherapy is a recent development in the management of patients with rectal cancer invading perirectal tissue and regional lymph nodes. This study was performed to assess the impact of preoperative adjuvant therapy in patients judged by endorectal ultrasound to have extramural invasion of rectal cancer and/or regional lymph node involvement on tumor regression in bowel wall T and lymph nodes N. The predictive value of ultrasound in staging wall penetration and lymph node involvement after preoperative adjuvant therapy was also assessed. METHODS Fifty-one patients were selected by ultrasound to have preoperative irradiation (40-50 Gy over 5-6 weeks). In 29 patients this was combined with 5-fluorouracil chemotherapy. Assessments of ultrasound were compared with pathologic findings in the resected specimen in all patients. RESULTS Partial downstaging was seen in 37 (72.5%) patients with wall invasion T and in five (9.8%) of 51 patients with lymph node involvement N. Complete downstaging was achieved in one (2.0%) patient with wall invasion T and in 20 (39.2%) of 51 patients with lymph node involvement N. Positive predictive values of ultrasound after irradiation were 47 (92.2%) and 45 (82.2%) for wall penetration and lymph node status, respectively. Negative predictive values of ultrasound after irradiation were rare 3.9% and 5.9%, respectively. CONCLUSIONS In the majority of patients with rectal cancer invading perirectal tissues or lymph nodes, lesions downstages by preoperative chemo radiotherapy. Endorectal ultrasound examination before and after chemo radiotherapy for rectal cancer is one of the most recommended in staging rectal cancer.