INTRODUCTION AND OBJECTIVE: Non-muscle invasive bladder cancer (NMIBC), a chronic condition, requires treatment and lifelong monitoring with regular endoscopic examinations. In this clinical context, symptom burden and quality of life are important patient reported outcomes (PROs) for consideration when making decisions about treatment. Current PRO measures inadequately assess NMIBC-specific symptoms and side effects of contemporary treatments. This study aimed to develop a patient-reported NMIBC Symptom Index (NMIBC-SI) acceptable to patients and clinicians and fit-for-purpose as a clinical trial endpoint. METHODS: We conducted a systematic review and interviewed 26 patients and 20 clinicians to develop a conceptual framework of PROs important to NMIBC. The 125 issues in the conceptual framework were phrased as questions and pre-tested in 12 cognitive interviews to develop a draft 104-item NMIBC-SI understood by and relevant to NMIBC patients. In Field Test 1 (FT1), we administered the draft NMIBC-SI to patients on active treatment from nine Australian sites. Items with low prevalence, conceptual similarity or high correlations (≥0.50) were flagged for exclusion. Nine Urologists agreed the final NMIBC-SI items. In Field Test 2 (FT2), patients from 16 sites across four countries completed the final version NMIBC-SI. This prospective longitudinal study evaluates the validity and reliability of the NMIBC-SI, assessing key PROs across treatments, disease trajectory, and risk categories. RESULTS: Planned accrual for FT1, n=220, was reached (178 male, mean age 69) representing Low 27.7%; Intermediate 13.2%; High 50.9% risk categories. 80% patients did not experience 21 items, 7 items were highly correlated, and 4 excluded as >50% of urologists rated them as not related to NMIBC treatment (e.g. coughing). The final 56-item NMIBC-SI used in FT2 included a 23-item symptom burden scale, 2 treatment specific modules, and 3 function scales. FT2 began July 2018. As of 30 Oct 2019, 168 patients have completed the NMIBC-SI before treatment (n=143), 1 week post-surgery (n=106), at end of induction therapy (n=72), and 1 year follow-up (n=10), across high (29%), intermediate (3%), and low (68%) risk categories. CONCLUSIONS: The NMIBC-SI allows comprehensive assessment of patients’ symptom burden and functioning impairment. The validated NMIBC-SI will be suitable to use in clinical trials of NMIBC treatments. In clinical practice, it will allow assessment of the impact of treatments for NMIBC on patients and allow greater patient involvement in the decision-making process. Source of Funding: Cancer Australia & Cancer Council NSW [PdCCRS Grant #1103036]