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Background The aim of the study was to evaluate the added value of the apparent diffusion coefficient (ADC) of diffusion-weighted magnetic resonance imaging (DW-MRI) in patients with rectal cancer who received neoadjuvant chemoradiotherapy (CRT). The use of DW-MRI for response evaluation in rectal cancer still remains a widely investigated issue, as the accurate detection of pathologic complete response (pCR) is critical in making therapeutic decisions. Patients and methods Thirty-three patients with locally advanced rectal cancer were evaluated retrospectively by MRI in addition to diffusion-weighted images (DWI) and its ADC pre- and post-neoadjuvant CRT. These patients subsequently underwent curative-intent surgery. Tumor staging by MRI and ADC value were compared with histopathological findings of the surgical specimen. Results MRI in addition to DWI had a sensitivity of 96.1%, specificity of 71.4%, positive predictive value of 92.5%, and negative predictive value of 83.3% in the detection of pCR. The pre-CRT ADC alone could not reliably predict the pCR group. Post-CRT ADC cutoff value of 1.49 x 10−3 mm2/s had the highest accuracy and allowed a 16.7% increase in negative predictive value and 3.9% increase in sensitivity. Patients with pCR to neoadjuvant treatment differed from the other groups in their absolute values of post-CRT ADC (p < 0.01). Conclusions The use of post-CRT ADC increased the diagnostic performance of MRI in addition to DWI in predicting the final pathologic staging of rectal carcinoma.