Background: Bladder recurrence after radical treatment of upper urinary tract urothelial cancer (UTUC) is frequent, and patients are required to undergo surveillance cystoscopies following surgery. The use of intravesical adjuvant chemotherapy is an accepted method to prevent bladder recurrence, but the timing of this method is not standardized and the concept of intraoperative use is unexplored. Objective: The objective of the study is to examine the feasibility and safety of intraoperative intravesical mitomycin C (MMC) instillation using a closed-circuit system following bladder cuff excision and bladder closure. Design, setting, and participants: All patients who underwent radical nephroureterectomy (RNU) for UTUC at the Department of Urology of Zealand University Hospital, Roskilde, Denmark from 2017 to 2020 were identified. Patient complications within 30 d and data regarding oncological outcome were registered. Outcome measurements and statistical ana lysis: Clavien-Dindo grade for complications and descriptive statistics were used. Results: During the study period, 64 patients underwent RNU. Of these patients, 49 received bladder instillation of MMC during RNU. Complications were observed in 11 patients (21.4%), where four patients (8.2%) had Clavien-Dindo complication grade (CD) I, four patients (8.2%) had CD II, one patient (2%) had CD III, and one patient (2%) had CD IIIa. None of the complications were suspected to be related to MMC. Five of the 15 patients (33%) who did not receive MMC experienced complications. There were no significant differences in complication rates between patients who received MMC and those who did not. Study limitations include a small sample size and a single-center study. Conclusions: Intraoperative vesical instillation of MMC is feasible and was, in the present study, not associated with an increased complication rate. Patient summary: Bladder recurrence after radical treatment of upper urinary tract cancer is frequent. The present study findings indicate that intraoperative bladder irrigation with the chemotherapeutic mitomycin C during surgery does not lead to excessive complications and could be a method to reduce the risk of bladder recurrence. (c) 2021 The Authors. Published by Elsevier B.V. on behalf of European Association of Urology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).