Objective: Demonstration of surgical steps of a Boari Flap ureteroneocystostomy in an oncological context. Methods: Clinical case of a 66-year-old woman diagnosed with a left-pelvic recurrence of a high-grade serous ovarian carcinoma, involving the left ureter. After transection of 5cm of ureteral length, up to the level of the bifurcation of common iliac vessels, it was decided to perform a Boari Flap for ureteral reimplantation. Results: Through the tubularization of a bladder flap, the extension of the ureter to the bladder is possible. After mobilization and psoas fixation, the bladder is opened on its anterior surface, in a rhomboid incision, and a full thickness bladder flap is extended cranially and tubularized for anastomosis of the proximal ureteral segment. The ureter is reimplanted after creation of an anti-reflux system with a submucosal tunnel between the mucosa and the detrusor. To finish the procedure, the bladder is closed in two layers with a running monofilament absorbable suture. Conclusions: Boari Flap ureteroneocystostomy is an uncommon urinary reconstruction, useful to correct distal ureteric defects caused by traumatic, oncological or iatrogenic injuries [1]. The success rate of ureteral reimplantation can be higher than 85% [2]. This technique is suitable for anastomosis of lumbar ureteral segments, allowing the better correction of higher and more extensive defects than the Psoas-Hitch technique. The flap length should compensate the ureteric defect and enable a tension free anastomosis.