Objective The KLIK method is an online tool that monitors and discusses electronic patient‐reported outcomes (ePROs), which has been shown to enhance outcomes. This study aimed (1) to determine the fidelity (ie, extent to which used as intended) of the KLIK method as implemented in outpatient pediatric cancer care and (2) to study health care professional (HCP)‐reported barriers and facilitators for implementation. Methods Two hundred five children with newly diagnosed cancer (enrollment rate 85%) participated. At 1 (T1), 3 (T2), and 6 (T3) months after diagnosis, patients (8‐18 years) or parents (of patients 0‐7 years) completed health‐related quality of life (HRQoL) questionnaires, which were transformed into an ePROfile and discussed by their HCP during consultations. Fidelity was determined by the following: percentage of website registrations, HRQoL questionnaires completed, and ePROfiles discussed. Implementation determinants were assessed with HCPs after the final T3 with the Measurement Instrument for Determinants of Innovations. Results Depending on the time point (T1‐T3), fidelity was 86% to 89% for website registration, 66‐85% for completed HRQoL questionnaires, and 56% to 62% for ePROfile discussion. Barriers were mainly related to organizational issues (eg, organizational change) and less frequently to users (eg, motivation to comply) or the intervention (compatibility). Facilitators were related to the user (eg, positive outcome expectations) and intervention (simplicity) but not to the organization. Conclusions When implementing ePROs in outpatient pediatric oncology practice, HCPs report determinants that influence ePRO integration. To improve implementation and outcomes, tailored organizational (eg, formal ratification by management and time) and specific local (eg, individualized assessments) strategies should be developed to achieve optimal ePRO discussion.