The aim of this study was to evaluate the correlation between the changes of SUV(max) and of apparent diffusion coefficient (ADC) before and after neoadjuvant therapy, to enable us predict the therapy response, in patients with locally advanced rectal cancer (LARC). A total of 30 patients with LARC who underwent CRT were recruited for our study. All the patients underwent a whole body 18F-FDG-PET/CT scan and a pelvic MR examination including DW imaging for staging (PET/CT1 and RM1), and after the chemoradiation therapy (PET/CT2, and RM2). Histopathologic analysis of rectal specimen, according to tumor regression grade (Mandard's criteria) was used as the standard reference. MR and PET-CT images were analyzed, and measurements of ADC values and SUV(max) were taken. Diagnostic performance for selection of complete responders (TRG1-2) and overall diagnostic accuracy for each item were calculated. After neoadjuvant therapy, all patients were submitted to surgery. According to Mandard's criteria, 21 tumors showed complete (TRG1) or subtotal regression (TRG2) and were classified as responders; nine tumors were classified as non responders (TRG3, 4, and 5). In all the patients, mean value of SUV(max) in PET/CT1 was higher than those in PET/CT2 (P < 0.001), whereas mean ADC value was lower in RM1 than RM2 (P < 0.001), with a significant percentage decrease of values after the treatment (P < 0.005).The best predictors cut-off values for TRG response were SUV(max) of 4.4 and ADC of 1.28 × 10(3) mm(2)/s with sensitivity, specificity accuracy, negative predictive value, and positive predictive values of 77.3%, 88.9%, 80.7%, 61.5%, and 94.4%, respectively. We conclude from the overall data of this study that the absolute values of SUV(max) and ADC of rectal lesion after CRT were the best parameters to define the response to treatment, by differentiating fibrosis from viable tumor tissue.