The widespread implementation of mammography screening has resulted in an increased frequency of needle core biopsies (NCB). The aim of this study was that of evaluating the diagnostic reproducibility on breast NCB, according to the B-classification, among several pathologists from different Italian regions. Fifty single slides of NCBs performed for non palpable breast lesions were selected to evaluate the diagnostic reproducibility, according to the B classification, among 31 pathologists from different Italian areas, involved in the pathologic diagnosis of screen-detected breast lesions. According to the study majority diagnosis (MD), 21 cases were classified as B2 (benign lesion), 23 B3 (lesion of uncertain malignant potential) and 6 B5 (malignant lesion). Overall, individual kappa coefficients in comparison to MD were good (mean 0.61, range 0.31-0.88). The level of inter-observer agreement, however, appeared lower in differentiating the two intermediate categories B2 and B3, thus potentially leading to over-treatment (false-positives: 26%) or under-treatment (false-negatives: 17%) of individual patients. Specific sub-types of B3 need an improvement of the diagnostic definition. A multidisciplinary approach and consultation with expert colleagues are recommended.