In a prospective investigation of the clinical and pathological effects of pelvic radiotherapy on the rectum, nine patients (age range 58-77 years) had symptoms assessed weekly during radical treatment for bladder and prostatic tumours, and at 2, 4 and 12 weeks after treatment. Stool frequency increased in all patients from a mean of 1.7 per day before treatment to 5.0 per day at 4 weeks (P < 0.05). Seven patients developed liquid stools during treatment; 6 had pain on defaecation, 8 had urgency of defaecation, all experienced tenesmus and 3 had episodes of faecal incontinence. One patient had symptoms of such severity that treatment was interrupted. Virtually all symptoms had resolved by 12 weeks after treatment. Sigmoidoscopy with biopsy of anterior and posterior rectal walls was performed before treatment began, at 2 weeks and 4 weeks during treatment, and 4 and 12 weeks after treatment. All patients during treatment had evidence of acute inflammatory damage with hyperaemic, oedematous mucosa and failure of healing of biopsy sites. Histological examination of the rectal biopsies revealed the well described characteristic mucosal changes with crypt cell damage, inflammatory cell infiltrate and loss of crypts. In 2 of 7 cases studied, pathology was still abnormal at 3 months. Radiation injury to bowel is an underemphasised cause of morbidity and further prospective studies are required to determine methods of reducing normal tissue damage and ameliorating symptoms.