The distal resection margin measured in situ at operation, on the fresh specimen and on the fixed specimen was analysed prospectively in 55 consecutive patients undergoing anterior resection for carcinoma of the rectum. There was obvious contraction in length from the operative margin to the specimen margin and to the fixed margin. As measured by contraction coefficients, contraction was greater in more proximal tumours than in distal ones. Eight of 55 patients had microscopic intramural spread in the fixed distal margin and the maximum spread was 12 mm. Taking contraction of the fixed margin into account, the extrapolated microscopic spread in the operative margin ranged from 0 to 47 mm.