BACKGROUND AND PURPOSE: MyHealth is a new follow-up program including individual nurse-led sessions based on Guided Self-Determination (GSD), which has been shown to improve health and psychological outcomes in patients after treatment for breast cancer. Fidelity assessment is important to support the implementation of GSD in clinical practice. The purpose of this study was thus to investigate fidelity and acceptance of the GSD program in the MyHealth study and whether sociodemographic and psychological factors were associated with patients' completion of the GSD program and completion of reflection sheets. MATERIAL AND METHODS: We assessed fidelity quantitatively by examining patients' completion of the GSD program (i.e. ≥3 sessions), completion of the reflection sheets and their associations with sociodemographic and psychological factors among 239 patients, and nurse-reported acceptance qualitatively through a focus group interview with all five nurses providing the GSD program. RESULTS: A total of 81% of patients completed the GSD program, while 71% of the reflection sheets were completed. Including a relative in a GSD session and lower education were significantly associated with completion of the program. Younger age and including a relative in a GSD session were significantly associated with completion of reflection sheets. Nurses found GSD highly applicable and especially appreciated a values-clarifying GSD reflection sheet and the inclusion of a relative. INTERPRETATION: The GSD program was applied with moderate-to-high fidelity, and the inclusion of relatives is potentially valuable. The GSD program indicates high usability and potential for being translated into clinical practice.