Objective. To describe the importance of the Statement of Assent for children and adolescents invited to participate in a clinical study and their main reactions to its explanation. Methods. This is an exploratory descriptive qualitative study of 17 children and adolescents, who were invited to participate in a clinical study in the field of oncology in a hospital located in Rio de Janeiro (Brazil). Data were analyzed using thematic analysis. Results. Two thematic units were generated after

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... interpretation: signing the statement of assent, in which participants felt their main role when faced with the possibility of expressing their agreement or not to take part in the study; and understanding of the study, when they showed that they understood the steps of the study by asking pertinent questions to clarify their doubts. Children and adolescents understood the steps of the study contained in the Statement of Assent, were interested and asked questions to clarify their doubts about the study. Conclusion. The Statement of Assent was important for participants understanding the study and having autonomy over their participation. As the statement strengthened the main role of children and adolescents in the research process, the conclusion was that its use in studies involving the pediatric population should be encouraged.