Oral mucositis (OM) is the most common local adverse event of chemotherapy treatment and leads to a debilitating condition from the patient’s perspective. The aim of this study is to report nine cases of severe OM over 10 weeks after initiating chemotherapy in pediatric patients with leukemia. All patients showed changes in normality of hematological data in most week. The most used chemotherapeutic agents were aracytin, etoposide and methotrexate. Saliva and lips were the most affected items. Well-designed observational studies are needed to better understand the risk factors for developing severe OM.