['1) To analyse for interchangeability of rigid sigmoidoscopy and MRI in determining the distance from anus to tumour, and to determine if anterior/posterior location influences this difference. 2) To analyse the effect of preoperative chemo-radiotherapy on the distance from anus to tumour.', 'Retrospective investigation of endoscopy reports and MRI series of 144 consecutive patients operated for rectal cancer.', 'The mean distance from the anal verge to the tumour measured by sigmoidoscopy was 82mm and by MRI 61mm (p<0.01). For tumours in the anterior quadrant this difference was 30mm and for tumours located in the posterior quadrant only 12mm. The distributions of the cancers as low, middle and high differ by more than 10% between the two methods. The coefficient of correlation between measurements was 0.9 but the variation was not acceptable. The length of the tumours decreased by 16mm after neoadjuvant treatment, but the distance from tumour to anus increased by only 4mm.'] Are rigid sigmoidoscopy and MRI interchangeable in determining the position of rectal cancers?