Objective: To evaluate the usefulness of intra-operative ultrasonography (US) for localization of nonpalpable breast cancer that could be visualized with preoperative US. Patients and methods: We prospectively assessed 57 nonpalpable sonographically detected and biopsy-proved breast cancers in 57 patients. US localization of breast cancers was performed in the OR by the radiologist immediately before definitive surgery using either injection of blue dye or placement of a guide wire after marking of the skin overlying the lesion with a marking pen. Tumor identification, the correlation with tumor diameter on preoperative US, analysis of resection margins, and the need to perform surgical re-excision were analyzed. Results: US correctly localized all lesions at surgery. Re-excision due to positive resection margins was necessary in four patients (7%) including three patients with ductal carcinoma-in situ (DCIS) and one patient with invasive disease at the surgical margin. Mastectomy was necessary in one patient (1.7%) due to multifocal invasive carcinoma. Thus, the re-excision rate was 8.7% (5 of 57). Conclusion: US in the operating room is an attractive alternative guiding tool of localizing nonpalpable breast cancers that have been seen on preoperative US improving the process of image-guided surgery.