['Recent studies have suggested that benign papillary lesions without atypia [benign papilloma (BP)] diagnosed on breast core needle biopsy (CNB) may not require excision. However, most have studied only small numbers of cases and scant data are available on the utility of immunohistochemistry in the categorization of papillary lesions on CNB. In the largest published series of BP identified on CNB, we studied the impact of immunohistochemistry on the accuracy of a CNB diagnosis of BP.', 'Breast CNBs (n = 129) with a diagnosis of papillary lesion were immunostained for calponin, p63 and cytokeratin 5/6. Haematoxylin and eosin and immunostained slides were independently reviewed by four breast pathologists. BP was the final excision diagnosis in 35 cases. With the use of immunohistochemistry, the positive predictive value (PPV) of BP diagnosis by the four individual pathologists increased from 72.7-83.3% (mean 79.2%) to 77.8-87.5% (82.1%), the negative predictive value (NPV) increased from 77.8-98.5% (88.6%) to 100% for all four participants and overall accuracy increased from 78.7-92.6% (84.7%) to 90.7-95.4% (92.8%). No case of invasive carcinoma was diagnosed as BP on CNB by any participant. The frequency of ductal carcinoma in situ following a BP diagnosis on CNB ranged from 2.5% to 4.8% (4%) but was only 0-3% (2.3%) after excluding cases that were radiologically suspicious for malignancy.'] Does immunohistochemistry increase the accuracy of diagnosis of benign papillary lesions in breast core needle biopsy specimens?