Patients given preoperative radiotherapy (31.5 Gy in 18 fractions) in a prospective, randomized trial of presumably operable rectal adenocarcinoma, were examined for a possible relation between bowel toxicity manifested as diarrhoea, and tumour size in the operative specimen, in addition to recurrence rate. The group requiring drugs for diarrhoea had significantly smaller tumours at surgery (2.5 cm versus 3.5 cm, p < 0.05). Patients without significant radiation-induced diarrhoea had also more recurrences (37.5% against 14.3%, p = 0.01). The disease-specific survival rate was also significantly better (p = 0.02) at 1.5 and 10 years in patients with diarrhoea WHO grade 3 and 4; 89.5%, 75.9% and 65.1% compared to 83.5%, 49.3% and 44.4% in patients with no or minimal radiation-induced loose bowels. These results indicate that the reaction of the normal bowel to radiation may correlate to radiation sensitivity of tumours derived from the same tissue.