The involvement of cancer patients in symptom reporting talks into our cultural narrative of empowerment and participation in decisions in health of both patients and professionals. Electronic patient-reported outcome (ePRO) is a tool applied for use in such interaction. Based on limited evidence and few empirical studies, health systems are rapidly implementing this instrument in managing patients in active treatment and in follow-up. In a cluster randomized trial of all Danish oncology departments treating breast cancer with adjuvant chemotherapy, we applied ePRO in 347 patients consecutively recruited. Our primary outcome, which was at least one treatment adjustment, was not significantly influenced using ePRO, 34% in the ePRO arm and 41% in the usual care arm received at least one treatment adjustment, p = 0.095. Number of hospitalizations and events of febrile neutropenia were not influenced by the intervention. We believe that one of the main reasons for this finding could be the application of PRO in a well-regulated treatment setting within a public health care system where the most impacting symptomatic toxicities are already taken care off. Trial registration: Clinicaltrials.gov identifier: NCT02996201, registered 19 Dec 2016, retrospectively registered.