OBJECTIVES: To determine the diagnostic value of T2-weighted signal intensity evolution in the tumour for detection of complete response to neoadjuvant chemoradiotherapy in patients with rectal cancer. METHODS: Thirty-nine patients diagnosed with locally advanced adenocarcinoma and treated with chemoradiotherapy (CRT), followed by surgery, underwent magnetic resonance imaging (MRI) before and after CRT on 1.5-T MRI using T2-weighted fast spin-echo (FSE) imaging. The relative T2-weighted signal intensity (rT2wSI) distribution in the tumour and post-CRT residual tissue was characterised by means of the descriptive statistical parameters, such as the mean, 95th percentile and standard deviation (SD). Receiver operating characteristic curves were used to determine the diagnostic potential of the CRT-induced alterations (Delta) in rT2wSI descriptives. The tumour regression grade (TRG) served as a histopathological reference standard. RESULTS: CRT induced a significant decrease of approximately 50% in all rT2wSI descriptives in complete responders (TRG1). This drop was significantly larger than for incomplete response groups (TRG2-TRG4). The DeltarT2wSI descriptives produced a high diagnostic performance for identification of complete responders, e.g. Delta95th percentile, DeltaSD and Deltamean resulted in accuracy of 92%, 90% and 82%, respectively. CONCLUSIONS: Quantitative assessment of the CRT-induced changes in the tumour T2-weighted signal intensity provides high diagnostic performance for selection of complete responders. KEY POINTS : * T2 weighted MRI helps predict response after chemoradiotherapy for rectal cancer. * Residual tumour and chemoradiotherapy-induced fibrosis have different T2 relaxation properties. * T2-weighted signal intensity evolution is a promising non-invasive marker of therapeutic response. * A pathologically complete response is associated with the largest signal intensity drop.