Background: Service user participation contributes to the improvement of health care services for the betterment of conditions experienced by patients. However, there is a lack of knowledge about how adolescents and young adults (AYAs) experience involvement processes in practice and what it would take to achieve a high degree of involvement according to AYAs. Objectives: To explore: (1) how AYAs with cancer experienced involvement processes during participation in three different service user involvement initiatives (development of an app, development of a book and youth panel meetings), and (2) AYAs' perceptions of determination and collaborative areas between AYAs and professionals. Methods: Individual semi-structured interviews were conducted with 12 AYAs with cancer aged 16-29 who had participated in at least one of the service user initiatives in Kræftværket, a youth cancer support centre and social organisation located at Copenhagen University Hospital, Denmark. We analysed data with thematic analysis. Results: We identified three main themes with matching sub-themes: Working procedures (Agenda setting, Workflow, Dialogue), Knowledge sharing (AYAs' needs and ideas, Areas of expertise), Influence (Decisions, Ownership). Generally, the participants experienced an equal dialogue with professionals and felt they had the final say on decisions. Conclusions: This study produced specific suggestions on how to involve AYAs to a high extent. Based on AYAs' own perceptions of involvement, AYAs', professionals' and shared determination areas were illuminated. Additionally, the AYAs highlighted how the professionals gave feedback during co-creation processes, which may inspire future service user involvement projects.