5-Fluorouracil (5-FU) is the most effective drug in gastrointestinal cancer. Mucositis and bone marrow toxicity are the two major limiting side effects. In our effort to reduce mucositis we administered 'Allopurinol' mouthwash in 16 patients who had experienced oral mucositis during prior treatment with 5-FU. In all patients significant reduction of oral toxicity was noticed as well as prolonged pain relief. This observation must be tested by controlled clinical trial.