OBJECTIVE: To develop a core outcome set (COS) applicable for effectiveness trials of all interventions for localised prostate cancer. Many treatments exist for localised prostate cancer, although it is unclear which offers the optimal therapeutic ratio; which is confounded by inconsistencies in the selection, definition, measurement and reporting of outcomes in clinical trials. PATIENTS, SUBJECTS AND METHODS: A list of 79 outcomes was derived from a systematic review of published localised prostate cancer effectiveness studies and semi-structured interviews with 15 patients with prostate cancer patients. A two-stage consensus process involving 118 patients and 56 international healthcare professionals (HCPs; cancer specialist nurses, urological surgeons and oncologists) was undertaken, consisting of a three-round Delphi survey followed by a face-to-face consensus panel meeting of 13 HCPs and eight patients. RESULTS: The final COS included 19 outcomes. In all, 12 apply to all interventions: death from prostate cancer, death from any cause, local disease recurrence, distant disease recurrence/metastases, disease progression, need for salvage therapy, overall quality of life, stress urinary incontinence, urinary function, bowel function, faecal incontinence, and sexual function. Seven were intervention-specific: perioperative deaths (surgery), positive surgical margin (surgery), thromboembolic disease (surgery), bothersome or symptomatic urethral or anastomotic stricture (surgery), need for curative treatment (active surveillance), treatment failure (ablative therapy), and side-effects of hormonal therapy (hormone therapy). The UK-centric participants may limit the generalisability to other countries, but trialists should reason why the COS would not be applicable. The default position should not be that a COS developed in one country will automatically not be applicable elsewhere. CONCLUSION: We have established a COS for trials of effectiveness in localised prostate cancer, applicable across all interventions that should be measured in all localised prostate cancer effectiveness trials.