Aims: Needle core biopsy (NCB) is a widely-used technique for non-operative evaluation of screen-detected breast lesions. Although most NCBs are B2 (benign) or B5 (malignant), some fall into the B3 category of “uncertain malignant potential”. This study aims to categorise the lesions prompting a B3 NCB in the Merrion Breast Screening Unit, and establish the incidence of malignancy on subsequent excision biopsy. Methods: Patients attending the Merrion Breast Screening Unit in Dublin between 2000 and 2008 who had a B3 NCB were identified. The NCB pathology reports were reviewed and the diagnosis correlated with excision histology; the latter was classified as benign, atypical or malignant. Lesion-specific positive predictive values (PPVs) for malignancy were derived. Results: 141 patients with a B3 NCB were identified. The most frequent lesions on NCB were radial scar (RS; n = 57), atypical intraductal epithelial proliferation (AIDEP; n = 25) and papillary lesion (n = 24). The final diagnosis was malignant in 22 patients (16%), atypical in 40 (28%) and benign in 79 (56%). Two of the patients with a malignant diagnosis had invasive carcinoma. The lesion-specific PPVs were: lobular neoplasia 50%, AIDEP 32%, columnar cell lesion with atypia 12.5%, RS 12.3%, papillary lesion 8.3%, suspected phyllodes tumour 7.7%, and spindle cell lesion 0%. Atypia on RS NCB predicted an atypical or malignant excision diagnosis, but atypia on papillary lesion NCB did not. Conclusions: One-sixth of B3 NCBs in this series proved to be malignant on excision. The PPV for malignancy varied according to lesion type.