Introduction: Breast lesions include a heterogeneous group of entities with variable clinical behaviour and morphological presentation, mostly classified as benign or malignant, with predictable behaviour. However, there are lesions with clinical, breast imaging and/or biopsy characteristics that do not clarify their nature. These lesions have an uncertain behaviour regarding their malignant potential at diagnosis.

We intend to relate the preoperative diagnosis of neoplasm of uncertain behaviour of the breast (NUnBB) regarding the core needle biopsy and the histological result after excisional biopsy.

Methods: This is a retrospective study of patients submitted to local excision of breast lesions with a perioperative diagnosis of NUnBB, classified as 2383 at ‘International Statistical Classification of Diseases and Related Health Problems’ (ICD 9), between January 2007 and October 2016 in a breast unit.

Results: Ninety-two cases with the diagnosis of NUnBB were analysed: 91 females with a mean age of 59 ± 14 years. All were submitted to local excision of breast lesion as ambulatory surgery with the following histology: 64% benign, 3% malignant potential and 33% malignant. Of those who presented malignant results, 69% underwent a surgical re-intervention for local control of the disease.

Discussion: Regarding the considerable number of malignant lesions at final histology and the high percentage of which are re-operated, NUnBB should be treated with the same priority as a confirmed malignant neoplasm and whenever possible, using the most appropriate surgical technique.