Objective: to evaluate the effect of oral cryotherapy compared to physiological serum on the development of oral mucositis in outpatient cancer patients using the 5-fluorouracil antineoplastic agent. Method: this is a controlled, randomized, double-blind, and multi-center clinical trial, conducted with 60 patients undergoing chemotherapy. The experimental group (n=30) used oral cryotherapy during the infusion of the 5-FU antineoplastic agent, while the control group (n=30) performed mouthwash with physiological serum at their homes. The oral cavity of the participants was assessed at three times: before randomization, and on the 7th and 14th days after using 5-FU. For data analysis, descriptive analyses and the ANOVA, paired t, and McNemar tests were used. Results: there was no statistically significant difference between the experimental and control groups in the assessments regarding the grade of mucositis. However, cryotherapy presented the chance to reduce the presence of intragroup mucositis, between the first and second assessments (p=0.000126). Conclusion: cryotherapy did not obtain statistical significance in relation to oral hygiene with serum, but it proved to be effective intragroup. Record number: RBR-4k7zh3