Background: For patients with intermediate- to high-risk non-muscle invasive bladder cancer (NMIBC), standard of care includes transurethral resection (TURBT) followed by induction and maintenance intravesical immunotherapy with Bacillus Calmette-Guerin (BCG). This study describes characteristics and clinical outcomes of Medicare beneficiaries newly diagnosed with NMIBC who initiated BCG therapy following TURBT. Methods: Retrospective cohort study using Centers for Medicare and Medicaid-sourced Medicare Fee-for-Service medical/pharmacy claims (100% sample). Patient selection criteria: ≥1 bladder cancer diagnosis 2010-2019, no prior radiotherapy, systemic therapy, cystectomy, or metastasis in 6 months post-diagnosis, continuously enrolled in Medicare ≥12 months pre- and post-index (or death). Index date = date of first BCG instillation. A literature-based definition for adequate maintenance BCG defined adequate (≥7 instillations of BCG within 274 days of BCG initiation) and inadequate (remainder) BCG therapy for patient stratification. Events: time to BCG discontinuation, radical cystectomy, systemic therapy, radiation, and metastasis. Provider access to BCG (due to supply shortage), patient tolerability, and NMIBC risk status are not captured in claims data. Results: Of 46,052 BCG-treated patients eligible for analysis, 19,859 (43.1%) received adequate BCG therapy. The two cohorts had similar baseline characteristics (Table) and median duration of follow up (46 months). Median (interquartile range [IQR]) time from NMIBC diagnosis to BCG initiation was 2.8 (8.2) months for adequate, 3.7 (14.4) months for inadequate BCG. Time from BCG initiation to discontinuation, radical cystectomy, systemic therapy, radiation, and metastasis are reported in the table. Conclusions: Between 2010 and 2019, 57% of Medicare patients initiating BCG following diagnosis of NMIBC and TURBT may not have received adequate BCG therapy. Patients with inadequate BCG therapy had numerically shorter time to radical cystectomy, systemic therapy, radiation, and metastasis than those with adequate BCG therapy. Characteristics & Outcomes by Cohort.