BACKGROUND AND PURPOSE In patients with locally advanced rectal cancer, neoadjuvant long course (45-54 Gy in 25-30 fractions) chemoradiotherapy (CRT) may reduce tumour size and result in downstaging. In patients with primary resectable tumour short course (25 Gy in 5 fractions) radiotherapy (SCRT) reduces local recurrence but downstaging the disease or altering tumour size has not been described. We aimed to assess change in tumour size on MRI after SCRT or CRT. MATERIALS AND METHODS Nineteen patients with rectal carcinoma underwent MRI before and after SCRT or CRT. In each case, tumour length and width were documented and number of locoregional lymph nodes recorded. Total mesorectal excision was performed in 15 patients and MR findings correlated with histopathology. RESULTS Ten patients received SCRT and nine CRT. Tumour length reduced by 19% overall (15% following SCRT, 23% following CRT). Greater than 30% reduction (partial response) in maximum tumour thickness was seen in 4/10 (40%) following SCRT and 5/9 (56%) following CRT. CONCLUSIONS Significant reduction in tumour size can be achieved with preoperative long course CRT and SCRT. This unexpected finding following SCRT has not been previously described.