Abstract Background The study is based on a national cluster randomized trial investigating the effect of electronic patient-reported outcomes (ePRO) on treatment outcomes in breast cancer patients receiving adjuvant chemotherapy. All 13 oncology departments (11 clusters) treating breast cancer patients in Denmark were randomized to use electronic patient-reported outcomes with real-time clinician feedback (ePRO arm) to track symptoms or usual care for eliciting symptoms using a short paper tracking list (usual care arm). The impact of ePRO on clinical outcomes were examined, which is reported elsewhere. The purpose of the present study was to examine patient-reported experience measure (PREM) regarding communication and handling of side effects/symptoms. Methods For this sub-study, patient representatives were involved in the development of a PREM questionnaire. Patients enrolled in the cluster randomized trial completed the PREM questionnaire at their last treatment visit. Semi-structured telephone-interviews were performed with a subgroup of patients. The interviews were based on an interview guide comprised of the questions from the PREM questionnaire and aimed to elaborate on the PREM questionnaire data. Results A 12 item PREM questionnaire was developed in partnership with patient representatives. In total, 439 out of 682 patients (64.4%) included patients completed the PREM questionnaire. Telephone semi-structured interviews were performed with 22 patients. In total, 52% (ePRO arm) and 65% (usual care arm) reported having talked with the oncologist/nurse about their responses in the tracking systems before each chemotherapy cycle. Fewer patients in the ePRO arm compared to the usual care arm experienced side effects/symptoms not included in the side effect questionnaire. Patients experienced high satisfaction with oncologists’ and nurses’ handling of side effects/symptoms. Conclusions Patients experienced high satisfaction with oncologists’ and nurses’ handling of chemotherapy adverse events. The study indicates a need for a more comprehensive side effect questionnaire as tracking system covering more symptoms than the one used in usual care today. Trial registration Clinicaltrials.gov identifier NCT02996201. Registered 19 December 2016, retrospectively registered.