Fifty rectal carcinomas resected during a 24-month period were examined by whole-mount giant sections. By light microscopy, the distance of maximal tumour invasion (penetration) into the bowel wall and the surgical clearance were measured by a new standardized method. The outer border of the muscularis propria and the line of surgical excision were used as the reference points. Preliminary results suggest that there is a significant inverse correlation between tumour penetration and radial surgical clearance. The adequacy of radial surgical clearance may affect the risk of local recurrence, and may prove useful as a prognostic indicator.