Resumo PURPOSE: Recent years have seen remarkable progress in cancer therapy, although treatment-induced adverse reactions and complications are not uncommon. Approximately 40 % of patients undergoing chemotherapy for cancer experience adverse reactions in the oral cavity, with nearly half of them developing severe oral mucositis that necessitates postponing therapy and/or changing the drug dosage. The objective of this study was to assess the usefulness of prophylactic professional oral health care (POHC) for preventing mucositis in patients undergoing chemotherapy. METHODS: Twenty-six female patients scheduled for chemotherapy for breast cancer were included in this study and randomized to the self-care or POHC groups. Assessment parameters included oral cavity photographs, plaque control records, Saxon test scores, Oral Assessment Guide scores, and grading using the Common Terminology Criteria for Adverse Events. Beginning before surgery and continuing through the completion of chemotherapy, the POHC patient group received weekly professional oral health care, including scaling, professional cleaning of the tooth surfaces, brushing instructions, and nutritional and lifestyle guidance. RESULTS: More patients in the self-care group developed oral mucositis than in the POHC group. The Oral Assessment Guide score, which was used as an index of oral mucositis, was also significantly lower in the POHC group. Based on the Oral Assessment Guide and plaque control records, there was almost no deterioration of the oral environment in the POHC group, whereas deterioration was observed in the self-care group. CONCLUSIONS: These findings demonstrate the efficacy of regular POHC in reducing the risk of oral mucositis in breast cancer patients undergoing chemotherapy.