Abstract Objective: To determine the occurrence and severity of oral mucositis (OM) and the factors associated with the onset of severe oral mucositis (SOM) in pediatric cancer patients. Material and Methods: This was a longitudinal and prospective study with 85 children and adolescents (aged 0 to 19 years) admitted to the Pediatric Oncology Sector of Napoleão Laureano Hospital between November 2016 and July 2019. The patients’ sociodemographic and epidemiological information, as well as hematological and biochemical parameters, were obtained from their medical charts. The oral mucosa of study participants was assessed through the modified Oral Assessment Guide (OAG) at baseline and after the 2nd, 5th and 10th week of antineoplastic treatment. Data were analyzed by Chi-square test to determine the factors associated with the occurrence of SOM (p<0.05). Results: The occurrence of OM was more frequent in the 2nd (60%), 5th (40.9%) and 10th week (43.2%) of antineoplastic treatment. During the follow-up period, SOM was more frequent in the 5th week (34.7%), followed by the 2nd (32.3%) and 10th weeks (29.7%). There was no association between the occurrence of SOM and the patient's age, type of tumor, chemotherapy regimen, and hematological and biochemical parameters. Conclusion: The occurrence of oral mucositis in pediatric cancer patients was significant, especially in the 2nd week of antineoplastic treatment. Severe oral mucositis was more prevalent in the 5th week of treatment and was not associated with any of the study variables.