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Purpose: Neoadjuvant radiochemotherapy (RCT) is an accepted treatment for locally advanced rectal cancer (LARC) that improves surgical outcomes. If a pathological complete response is achieved, conservative surgery can be considered. The objective of our study was to assess the reliability of (18)F-FDG PET/CT for evaluating the response to neoadjuvant RCT in LARC., Methods: We prospectively studied 41 patients diagnosed with LARC and candidates for neoadjuvant RCT. PET/CT was performed before RCT and again 7 weeks later. A visual and semiquantitative analysis was carried out. The pathological response was classified according to the Mandard tumour regression grade (TRG). We analysed: (a) the relationship between TRG and the result of the posttreatment PET/CT scan, and (b) the correlation between the percentage of pathological response and the percentage decrease in SUVmax according to the response index (RI)., Results: The mean SUVmax of the rectal lesions at diagnosis was 13.6 and after RCT 3.96. The mean RI was 65.32 %. Sensitivity was 88.88 %, specificity 92.86 %, positive predictive value 96 %, negative predictive value 81 %. Of the 41 patients, 8 had TRG I (all negative PET/CT); 6 had TRG II (5 negative, 1 positive PET/CT); 16 had TRG III (13 positive, 3 negative PET/CT); 9 had TRG IV (all positive PET/CT); 2 had TRG V (all positive PET/CT). Of the 14 patients classified as responders (TRG I, II), 13 (92.86 %) had negative PET/CT. Of the 27 patients classified as nonresponders (TRG III-V), 24 (88.88 %) had positive PET/CT. Differences were statistically significant (p < 0.0001). The RI in responders was 79.9 % and in nonresponders was 60.3 %. Differences were statistically significant (p < 0.037)., Conclusion: PET/CT is a reliable technique for assessing response to neoadjuvant RCT in LARC, with a view to considering more conservative surgical treatment. The combination of the visual and semiquantitative analysis increases the diagnostic validity of PET/CT.