Background and Aims Nephron sparing surgery (NSS) is a standard technique for patients with bilateral Wilms tumor (WT) and unilateral WT with predisposing syndromes. The decision for intra-operative ureteral stent placement depends on the degree of disruption of the collecting system and must be balanced between the risk of post-operative urinary leak and the risk of stent-related urinary tract infection. The purpose of this study is to find predictors of urinary leak that may guide the decision for ureteral stenting. Methods Patients who underwent NSS for pre-operative diagnosis of renal masses at a single tertiary pediatric hospital between January 2010 and December 2023 were retrospectively reviewed. Exclusion criteria were post-operative diagnosis of non-neoplastic conditions and incomplete data. The following pre-operative variables were studied: laterality of the tumor, symptoms at diagnosis, timing of surgery, pre-operative chemotherapy toxicity grade 3 or higher according to the Common Terminology Criteria for Adverse Events (CTCAE; version 5.0). Three intra-operative variables were studied: the need for intra-operative repair of distal calyces, total duration of surgery and duration of vascular clamping. Primary outcome was the occurrence of urinary leak. Secondary outcomes were post-operative renal dysfunction and post-operative length of hospitalization. Results Thirty-seven patients underwent NSS; seven of them were excluded for post-operative diagnosis of non-neoplastic disease or incomplete data. Twenty patients had unilateral disease and ten had bilateral disease or tumor on horseshoe kidney. Eight patients underwent upfront surgery and twenty-two received neoadjuvant chemotherapy. Ureteral stent was placed intra-operatively in three patients (10%). Urinary leakage was observed in four patients (13%) who did not undergo intraoperative stenting; all of them required delayed stent placement. Urinary leak was significantly associated with pre-operative chemotherapy toxicity (4/10 patients versus 0/20 patients, p = 0.0077). Post-operative renal dysfunction was significantly associated with pre-operative chemotherapy toxicity (8/10 patients vs 3/20 patients; p = 0.0010), bilateral disease (7/10 patients vs 4/20 patients; p = 0.0147) and surgery after neoadjuvant chemotherapy (11/22 patients vs 0/8 patients; p = 0.0140). Post-operative hospitalization was significantly longer in patients with pre-operative chemotherapy toxicity (median 10.5 days versus 7 days, p = 0.0255) and in patients who underwent surgery after neoadjuvant chemotherapy (median 9.5 days versus 6.5 days, p = 0.0053). Conclusions Patients with a history of severe pre-operative chemotherapy toxicity have a higher risk of post-operative urinary leak after NSS. In these patients planned intra-operative ureteral stenting should be considered.