Background: Patient-reported outcomes (PROs) are getting widely implemented, but little is known of the impact of applying PROs in specific cancer diagnoses. We report the results of a randomized controlled trial (RCT) of the active use of PROs in patients with locally advanced or metastatic bladder cancer (BC) undergoing medical oncological treatment (MOT) with focus on determining the clinical effects of using PROs during chemo- or immunotherapy compared to standard of care. Methods: We recruited patients from four departments of oncology from 2019 to 2021. Inclusion criteria were locally advanced or metastatic BC, initiating chemo- or immunotherapy. Patients were randomized 1:1 between answering selected PRO-CTCAE questions electronically once weekly with a built-in alert-algorithm instructing patients of how to handle reported symptoms as a supplement to standard of care for handling of side effects (intervention arm (IA)) vs standard procedure for handling of side effects (control arm (CA)). No real-time alerts were sent to the clinic when PROs exceeded threshold values. Clinicians were prompted to view the completed PROs in the IA at each clinical visit. The co-primary clinical endpoints were hospital admissions and treatment completion rate. Secondary endpoints were overall survival (OS), quality of life (EORTC’s QLQ-C30 and QLQ-BLM30) and dose reductions. Results: 228 patients with BC were included, 76% were male. 141 (62%) of the patients had metastatic disease. 51% of patients in the IA completed treatment vs. 56% of patients in the CA, OR 0.83 (95% CI 0.47–1.44, p = 0.51). 41% of patients in the IA experienced hospitalization vs. 32% in the CA, OR 1.48 (95% CI 0.83–2.65, p = 0.17). OS was comparable between the two arms (IA: median 22.3mo (95% CI 17.0-NR) vs. CA: median 23.1mo (95% CI 17.7-NR). Patient and clinician compliance was high throughout the study period (80% vs 94%). Conclusions: This RCT did not show an effect of PRO on completion of treatment, hospitalizations or OS for BC patients during MOT despite a high level of patient and clinician compliance. The lack of real-time response to alerts remains the greatest limitation to this study.