Objective: To assess patient adherence to intravesical instillation therapy for nonmuscle invasive urothelial carcinoma outside of clinical trials., Materials and Methods: We reviewed the records of patients from 2000 to 2013 who received intravesical therapy for nonmuscle invasive urothelial carcinoma. Patients with evidence of tumor recurrence or progression were excluded. We performed univariable and multivariable regression analyses to predict adherence to intravesical therapy., Results: A total of 729 patients started 861 induction cycles, 63% with bacillus Calmette-Guèrin (BCG) and 37% with mitomycin C (MMC). The rate of completion of 6 weeks induction therapy with BCG and MMC was similar (86% and 87%, respectively). Within the BCG cohort, 161 (35%) patients commenced the Southwest Oncology Group (SWOG) maintenance protocol after induction and 16 (10%) completed all 21 treatments. A monthly protocol for BCG was started by 87 patients (19%) and 48 (55%) completed all 9 treatments. MMC therapy was started in 270 patients, 97 of whom (36%) commenced monthly maintenance treatment, and 46 (47%) completed treatments. Median number of instillations was 7 for patients undergoing monthly maintenance therapy (MMC or BCG) and 9 for patients allocated to 3 years BCG. On multivariable analysis, recurrence after prior treatment of urothelial carcinoma was predictive of patients' adherence to treatment., Conclusion: Compliance with intravesical therapy is low in clinical practice, notably for longer treatment schedules., (Copyright © 2018 Elsevier Inc. All rights reserved.)