Abstract Objective This study aims to test the effectiveness on psychosocial outcomes of electronic feedback knowledge of radiotherapy intervention (e-Re-Know) for breast cancer patients. Method Randomized controlled trial in one university hospital in Finland was carried out. Breast cancer patients (n = 126) in the radiotherapy (RT) department were randomly assigned into two groups: intervention (the e-Re-Know and standard education) and control group (standard education). The e-Re-Know intervention consisted of e-feedback after response to the knowledge test delivered by e-mail. Instruments were completed before commencing first RT (M1), after concluding last RT (M2) and 3 months after last RT (M3). The main outcomes were anxiety and QOL. Results Compared with the control group, the patients in the intervention group reported a marginally significant improvement in anxiety and significant improvement in QOL over time. Conclusion The e-Re-Know seems to have positive effects on psychosocial outcomes for breast cancer patients. They might gain additional value from the e-Re-Know over a longer time period. Further research needs to focus more on development of e-feedback in patient education.