OBJECTIVE Haemorrhagic radiation-induced proctitis is a serious complication of radiotherapy of pelvic organs. In severe cases, massive haemorrhage may necessitate hospitalization and repeated transfusions. Application of formaldehyde under direct vision is one of the most efficient treatments. The aim of this study was to evaluate the results of this treatment as well as the histological changes induced by formaldehyde on the rectal mucosa. METHODS From January 1991 to September 2001, 13 patients who presented a haemorrhagic radiation-induced proctitis have been treated in our outpatient clinic with 4% formaldehyde cotton soaked applications. They were followed up to one year after the treatment. Endoscopic biopsies were performed before, immediately after the application, 1 month later, as well as at the one-year follow-up. RESULTS In eight cases bleeding stopped after the first application. In two patients a second application was necessary to control the haemorrhage and in two other patients bleeding ceased definitively after the fourth application. Follow-up evaluation at 12 months showed no sign of acute proctitis or rebleeding. One asymptomatic patient had a mild stenosis of the rectum. Baseline biopsies showed signs of acute inflammation. Those performed after the application of formaldehyde showed fresh thromboses of the vessels of the mucosa. Biopsies at 1 month and 1 year showed only chronic changes secondary to the radiotherapy. CONCLUSIONS Local application of 4% formaldehyde for the treatment of haemorrhagic radiation-induced proctitis gives good results, is well tolerated and easy to perform. Formaldehyde applied selectively causes thromboses of the bleeding vessels, without deep lesions or extended necrosis.