Pursuant to a promising report suggesting that an allopurinol mouthwash could have a protective effect against 5-fluorouracil (5-FU)-induced stomatitis, the authors performed a randomized, placebo-controlled, double-blind, crossover study. Seventy-seven patients, receiving their first 5-day course of chemotherapy with 5-FU +/- leucovorin, were assigned to use a mouthwash containing 20 mg of allopurinol or a placebo. The mouthwash was administered every hour for four doses commencing with each chemotherapy dose. The severity of subsequent mucositis was graded (on a 0-4 scale) by the attending physician and also by a patient-completed questionnaire. There was trend toward less mucositis in the placebo group with mean physician-judged mucositis scores of 1.3 for placebo and 1.8 for allopurinol (P = 0.07) and mean patient-judged mucositis scores of 1.5 for placebo and 1.9 for allopurinol (P = 0.15). There were no substantial differences in mucositis attributable to the two mouthwashes in the patients who crossed-over on their second cycle of chemotherapy. These data demonstrate that the tested allopurinol mouthwash regimen does not offer any protective effect against 5-FU-induced mucositis.