PURPOSE: Preoperative chemoradiation treatment (CCRT) for locally advanced rectal cancer has been known to be safe and effective. The aim of study is to find any correlation between tumor volume reduction and histopathologic downstaging in locally advanced rectal cancer by preoperative CCRT. METHODS: A total of 16 patients of rectal cancer were selected. They had been T3,4 N (+) preoperatively staged by using a transrectal ultrasonography and pelvic MRI. Radiation was given, a total of 5,040 cGy over 5 weeks, and systemic chemotherapy was also given 5 FU 450 mg/m2 and leucovorin 20 mg/m2 concurrently intravenously during the first and the fifth week of CCRT. Surgery was done 4~6 weeks after completion of CCRT. A 3D CT image was obtained with AcQsim PQ 5000 3D (Philips, USA). Tumor volume was measured before and after CCRT. RESULTS: The type of operative procedures were abdominoperineal resection 7, low anterior resection 5, coloanal anastomosis 3 and Hartmann operation 1. Volume reduction was ranged from 14.6% to 84.4%. Over a 50% tumor volume reductions were in 9 patients (56.2%). Pathologic complete remission was observed in 2 patients (12.5%), who showed 72% and 58.5% tumor volume reductions. Patients showing pT and/or pN downstaging patients (N=9) had a 55.9% tumor reductions (14.6~84.4%), and patients showing no pT and/or pN downstaging (N=7) had 51.8% volume reduction (24.7~79%). CONCLUSIONS: Preoeperative CCRT has been thought to be able to decrease tumor size and volume and to increase respectability. However, among our 9 patients who showed over 50% tumor volume reductions, 3 patients did not show any T and N downstaging, which is really important for long-term oncologic outcomes.