BACKGROUND: It is known that chemotherapeutic agents are not equally stomatotoxic and oral cavity lesions are the most frequent complications encountered in antineoplastic chemotherapy. AIMS: The objective of this study was to evaluate the occurrence of severe oral mucositis during a chemotherapy treatment and to identify its relationship with the chemotherapeutic class used. MATERIALS AND METHODS: This is a longitudinal, prospective, and observational study that used an intensive direct observation technique for assessing the oral clinical conditions and the chemotherapy treatment administered to 105 patients (both children and adolescents). RESULTS: = 0.012). CONCLUSION: It was concluded that the chemotherapeutic agents most related to severe oral mucositis and to the interruption in chemotherapy are those of the class of antimetabolites, especially the methotrexate and the Ara C. HOW TO CITE THIS ARTICLE: Chemotherapy in Pediatric Oncology Patients and the Occurrence of Oral Mucositis. Int J Clin Pediatr Dent 2019;12(4):261-267.