Background and purpose: Acute radiation-induced diarrhea occurs in approximately 80% of the patients receiving pelvic radiotherapy. It is caused by gastrointestinal irritation and inflammation. Eicosanoids are thought to be one of the mechanisms of this. Sulphasalazine is an inhibitor of their synthesis in the mucosa. This randomized clinical trial was undertaken to evaluate its effect in preventing acute radiation enteritis (ARE).