Eight patients with colon perforations after combined treatment with radiotherapy and steroid medication are studied retrospectively. Sigmoid diverticuli perforated after a minimum of 5 days treatment. The main symptom was a varying degree of abdominal pain. Clinical parameters were of limited value. The most important single examination was plain abdominal x-ray, though this was not positive in all cases. Seven patients were operated on, one with simple suture, the rest with Hartmann's procedure. None developed postoperative peritonitis. One died from cerebral infarction. Four died within 38 days postoperatively after discharge from hospital. Abdominal symptoms during high dose steroid treatment may indicate perforation of the colon. Immediate operative intervention may have an uncomplicated postoperative course.