Fifty consecutive specimens, obtained in the course of potentially curative abdominoperineal resection for rectal carcinoma situated 5–10 cm from the anal verge, were examined for the presence of microscopic distal intramural spread. Thirty-eight patients (76 per cent) were found to have no distal intramural spread. Seven patients (14 per cent) had spread for 1 cm or less and only 5 patients (10 per cent) had spread of more than 1 cm. Each of these 5 patients had a poorly differentiated Dukes' C carcinoma and each was dead or dying from distant metastases within 3 years of the operation. The results of anterior resection for carcinoma of the rectum were reviewed a minimum of 5 years after operation, to find out whether patients with a wide distal margin of resection had fared better than patients with a small margin. Seventy-nine patients had undergone a potentially curative resection, 48 with a distal margin of less than 5 cm (mean 2·8 cm; group 1) and 31 with a distal margin greater than 5 cm (mean 6·5 cm; group 2). The two groups were well matched for age, sex, degree of differentiation of the tumours and distance of the lesion from the anal verge, but 54 per cent of patients in group 1 had Dukes' grade C tumour whereas only 23 per cent of the patients in group 2 had Dukes' C tumours. Despite the higher proportion of unfavourable tumours in group 1, the outcome, in terms both of survival and of recurrence, was as good in the patients with the small distal margin as in the patients with the wide distal margin of clearance. The rigid, routine application of the 5 centimetre rule' of distal excision may cause patients with low rectal cancer to lose their anal sphincter unnecessarily.