Childhood cancer is one of the main causes of child mortality and its treatment has debilitating effects on the oral cavity. Several oral mucositis (SOM) is one of the most common and may cause undesirable symptoms such as pain and risk of systemic infection. This was a longitudinal, retrospective, and observational study determining the incidence of severe oral mucositis (SOM) and its occurrence sites in pediatric oncologic patients, in João Pessoa, Brazil, between 2013 and 2018. Data from 56 patients aged 1 to 18 years were collected from their medical records and through an oral mucosa examination, from the 1st to 5th week of chemotherapy treatment (CT) using the modified Oral Assessment Guide, by previously calibrated examiners (Kappa index > 0.7). The data were analyzed by the Chi-square test, and Odds Ratios were calculated. Most patients were females (54.5%), aged 8.8 years (± 4.8), with hematologic tumors (73.2%), predominantly Acute Lymphoid Leukemia (50.0%). An increase in the occurrence of SOM was observed throughout the CT (P = 0.05), ranging from 12.5% in the 1st to 35.7% in the 5th CT week. In the 1st CT week, there was a predominance of alterations in the lips (5.5%) and saliva (5.5%), while in the 5th, the jugal / palate mucosa (21.4%) remained the most affected site by SOM. Differences in the severity of SOM in the jugal / palate mucosa (P = 0.01) and labial mucosa (P = 0.04) were observed over time. In the 5th CT week, the likelihood of developing SOM was 13.3-fold higher (95% CI: 1.5 - 105.6) in patients with hematologic tumors. The incidence of SOM was higher in the 5th CT week, most commonly affecting the jugal / palate mucosa, and patients with hematologic tumors were more prone to develop SOM.