5-Fluorouracil (5-FU) is the most commonly used drug in carcinomas of the gastrointestinal tract. However, stomatitis is one of the limiting side effects. In the present study, 18 patients with gastrointestinal cancer who experienced 5-FU-induced stomatitis in previous cycles used allopurinol mouthwashes in subsequent courses of 5-FU (500 mg of 5-FU/m2/day, i.v., over 2 h for 5 days). The degree of stomatitis diminished in 15 patients (83.3%). Allopurinol mouthwash could be used as a simple and effective method for the reduction of 5-FU-induced oral toxicity. However, further controlled trials are required.