A randomized, double-blind, placebo-controlled trial was conducted to evaluate the effectiveness of a sucralfate mouthwash in preventing and alleviating oral mucositis induced by 5-fluorouracil (5FU). A total of 81 patients with colorectal cancer were enrolled. Patients were studied during their first cycle of chemotherapy with 5FU and leucovorin (LV) daily for 5 days every 4 weeks (Mayo Clinic schedule). Patients were randomly allocated to receive either a sucralfate suspension or a placebo suspension that was identical in appearance. Patients were instructed to use the suspension as a mouthwash four times daily from the beginning of the chemotherapy cycle. All patients received oral cryotherapy. Patients graded the severity of their own symptoms on a daily basis, and this was the primary outcome measure. There was no difference in the frequency or severity of oral mucositis between the sucralfate- and the placebo-treated group. Some mucositis was reported by 79% of the patient group. Assessment of mucositis by trial staff underestimated the incidence of this problem. Results of this trial do not support the hypothesis that a sucralfate mouthwash can prevent or alleviate oral mucositis induced by 5FU. Patient reporting of mucositis is a more sensitive instrument for assessment of mucositis than review by medical staff.