In 52 patients with rectal adenocarcinoma whole-mount sections of the entire operative specimen were examined by transverse slicing. There was spread to the lateral resection margin in 14 of 52 (27%) patients and 12 of these proceeded to local pelvic recurrence. The specificity, sensitivity, and positive predictive values were 92%, 95%, and 85%, respectively. In a retrospective stage-matched and grade-matched control group there was local recurrence in the same proportion of patients, but in this series no patient had been shown by routine sampling to have lateral spread. In rectal adenocarcinoma, local recurrence is mainly due to lateral spread of the tumour and has previously been underestimated.