We investigated the efficacy of diffusion-weighed whole body imaging with background body signal suppression (DWIBS) in assessing effects of chemoradiation therapy (CRT) on rectal carcinoma. DWIBS was performed in patients (n=12) with primary rectal carcinoma undergoing preoperative CRT before and 3 weeks after the treatment. Each patient received a total irradiation dose of 45 Gy at a single dose of 1.8 Gy administered once daily. Parallel to this, in the 1st, 3rd and 5th weeks 350 mg/m2 5-fluorouracil and 35 mg/m2 l-leucovorin were administered for 5 days. The apparent diffusion coefficient (ADC) was measured by DWIBS and surgical resection of the tumors enabled a correlation of ADC values with the pathological findings. With respect to histopathological grading of regression, two, five and five cases exhibited Grade 3, Grade 2 and Grade 1, respectively. In all patients, ADC values were higher after completion of CRT compared to those before it( 1.23+/-0.26x10(-3) mm2/s vs 0.75+/-0.13x10(-3) mm2/s, p<0.001). After completion of CRT, mean ADC values were 1.71+/-0.38x10(-3) mm2/s, 1.25+/-0.10x10(-3) mm2/s and 1.02+/-0.08x10(-3) mm2/s for Grade 3, Grade 2 and Grade 1, respectively. These preliminary results indicate that DWIBS may be a valuable tool to assess effects of CRT on rectal carcinoma by using appropriate cut-off values.