Introduction: Normal cells are also affected during antineoplastic treatment, including oral mucosa cells, which potentially causes oral complications.Objective: This study aimed to assess the prevalence of the main oral complications and the oral hygiene index in children and adolescents aged 5 to 12 years, who received chemotherapy and/or radiotherapy. Methods:It is an observational prevalence study performed in a hospital in Southern Brazil. Data were collected from 21 hospitalized patients with a self-application questionnaire for the responsible persons of children and with an intraoral clinical examination. The indicators used were DMFT (decayed, missing, and filled teeth) for dental caries, SOHI (simplified oral hygiene index) for oral hygiene, and the presence of mucositis, xerostomia, and candidiasis.Results: Most of the sample was male (57.1%) with average age of 8 years (sd 2.92), diagnosed with leukemia (47.6%). All patients received chemotherapy as treatment of choice and 38.1% received radiotherapy as part of the treatment. The results showed the presence of 61.9% of mucositis in patients and 28.6% of xerostomia. No patient presented candidiasis during clinical examination. As for dental caries, 66.7% of patients showed a DMFT from 4 to 16, which is considered high, and 38.1% of patients showed the presence of bacterial plaque and dental calculus. Conclusion:It is concluded that the main oral complications in children during antineoplastic treatment were mucositis and xerostomia. It was observed a high rate of dental caries as a consequence of an inadequated oral hygiene. Open acess What is the purpose of this study?The present study aimed to evaluate the prevalence of the main oral complications and the oral hygiene index in children and adolescents aged 5 to 12 years submitted to chemotherapy and / or radiotherapy in order to demonstrate the importance of the dental surgeon as a member of pediatric multidisciplinary oncological team. What researchers did and found?Oral clinical examination was carried out in patients undergoing cancer treatment. It was reported that the main oral complications in children and adolescents during the antineoplastic treatment were mucositis and xerostomia. Regarding oral health conditions, children and adolescents have a high caries rate, often associated with brushing difficulties due to oral complications of treatment. What do these findings mean?Due to the presence of oral problems related to the treatment of cancer, the presence of the dentist in the oncological team is vital for reducing the morbidity of these lesions. Longitudinal and full-time monitoring of the pediatric patient is necessary because of the risk of recurrence of some complications, even after ending the antineoplastic treatment.