jats:p Background: Nonpalpable breast lesions require localization, the gold standard for which is preoperative ultrasound-guided wire localization (PUGWL). Our unit also employs intraoperative ultrasound-guided wire localization (IUGWL). Here we evaluate PUGWL and IUGWL outcomes between 2014 and 2018. Primary outcomes were reoperation rates, complication rates and average specimen weights. Trainee feedback and cost analysis assessed IUGWL viability. Methods: Prospectively recorded data were collected. 511 patients were included (241 PUGWL and 270 IUGWL). Results: Reoperation rates: PUGWL 17.7% versus IUGWL 13.9% (p = 0.28) . Complication rates: PUGWL 5.8% versus IUGWL 6.6% (p = 0.72) . Average specimen weight: PUGWL 34.2 g versus IUGWL 24.3 g (p < 0.0001) . Trainees needed 15 supervised cases to be IUGWL competent. Performing IUGWL saves £289 per localization. Conclusion: IUGWL outcomes are comparable to those of PUGWL. IUGWL is cost-effective, patient-friendly and easy to learn and replicate. IUGWL merits wider dissemination and further planned research. </jats:p>