Background B3 breast lesions are a heterogeneous group with uncertain malignant potential and, as such, provide a source of diagnostic difficulty. We calculated the prevalence of B3 lesions at our center along with the upgrade rates (positive predictive value) to in situ or invasive malignancy. Materials and Methods We searched our pathology database over a 3-year period to include all B3 biopsies. The subsequent excision for each biopsy was reviewed, and the rate of upgrade was calculated by subtype. These results were compared against data published in large United Kingdom studies. Results A total of 9206 breast biopsies were identified, of which 614 (6.7%) were classified as B3. Lesions displaying epithelial atypia were the most common subtype of lesion, with a prevalence of 39.6%. Lesions displaying epithelial atypia were upgraded to malignancy in 35.7% of cases. Among non-atypical cases, papillary lesions were the most common diagnosis (32.1%) with an upgrade rate of 2%. In situ lobular neoplasia (10.4%) was the third most frequently encountered diagnosis, and was upgraded to malignancy in 10.9% of cases. The upgrade rate in the remaining non-atypical lesions was invariably low (0%-2.6%). Conclusions Herein, we have shown an overall B3 rate in keeping with published data, whereas lesions displaying epithelial atypia showed upgrade rates to malignancy comparable with that of large United Kingdom studies. In our study, lesions without epithelial atypia showed very low rates of upgrade. A wide range of upgrade rates is seen in cases of lobular neoplasia, which highlights the need for uniformity of nomenclature and reporting within this subtype to accurately ascertain the true risk of upgrade associated with these lesions.