Background: B3 breast lesions identified on core needle biopsy have uncertain malignant potential. Traditional management of these lesions has been surgical excision, but there is growing interest in less invasive and more cost-effective alternatives such as vacuum-assisted excisional biopsy (VAEB). Determining the rate of malignant upgrade for B3 lesions is important as it may identify low-risk lesions where VAEB could be considered. Methods: A retrospective study was conducted of women undergoing an elective excisional biopsy for a B3 lesion identified on core needle biopsy at a tertiary Australian breast centre. The pre-operative biopsy diagnosis and subsequent excisional biopsy diagnosis were used to calculate the proportion of cases where the diagnosis was upgraded to malignancy. Results: A total of 299 eligible patients were identified. Pre-operative diagnosis of papillary lesion with atypia was associated with the highest upgrade rate (50%, n = 12). The next highest upgrade rates occurred in those with flat epithelial atypia (37.50%, n = 8); atypical ductal hyperplasia (24.71%, n = 85); lobular carcinoma in situ (LCIS)/atypical lobular hyperplasia with calcification (17.65%, n = 17); and papillary lesion without atypia (4.72%, n = 106). Patients with radial scar (n = 51), classical LCIS without calcification (n = 7) and mucocoele-like lesion (n = 8) had a 0% upgrade rate. Conclusion: VAEB may be appropriate for low malignant risk lesions such as papillary lesion without atypia, mucocoele-like lesion and radial scar lesion without atypia. Open-surgical-excisional biopsy remains appropriate for high upgrade lesions such as atypical ductal hyperplasia, papillary lesion with atypia, flat epithelial atypia and classical LCIS with calcification. Long-term prospective randomized multicentre studies and continuing multidisciplinary approach is recommended for future clinical implementation.