Luminal cytokeratin (CK) expression in breast papillary lesions, and its potential diagnostic utility among other markers in distinguishing between papillomas and papillary carcinomas, has not been previously evaluated. Such expression was determined in 42 papillary lesions (18 papillary carcinomas and 24 papillomas) by immunostaining with a CK5/p63/CK8/18 antibody cocktail. The mean CK8/18 intensity score and percentage of positive cells were significantly higher in papillary carcinomas (227 and 95%, respectively, vs 86 and 42% in papillomas; both P-values o0.0001), whereas the mean CK5 intensity score and percentage of positive cells were significantly lower (7 and 5%, respectively, vs 107 and 58% in papillomas; both P-values o0.0001). Half (9/18) of the papillary carcinomas expressed p63 vs all (24/24) of the papillomas (P ¼ 0.0001). P63 expression in papillary carcinoma was always (9/9; 100%) focal/limited in nature (expression in o10% of cells), whereas focal expression was seen in only four (17%) papillomas (Po0.0001). Both differential CK (CK8/18 and CK5) expression and p63 were equally sensitive (100%) for the diagnosis of papillary carcinoma, but differential CK expression was more specific (96 vs 83%), resulting in a greater accuracy. However, the best discriminatory power in the distinction from papilloma was achieved when all three markers were used in combination, resulting in 100% sensitivity and specificity values. It is concluded that breast papillary lesions have differential CK expression profiles that, especially in combination with p63, can be useful for their stratification, potentially also in needle biopsy material, in which more accurate and reproducible characterization is needed. Modern Pathology (2011) 24, 185-193; doi:10.1038/modpathol.2010; published online 12 November 2010 Keywords: breast; cytokeratin 5; cytokeratin 8/18; luminal cytokeratin; p63; papillary carcinoma; papilloma By comprising only 1-2% of breast lesions, 1 papillary lesions are relatively rare. Despite that, recognizing that a breast lesion is papillary in nature is not particularly difficult for pathologists. On the other hand, distinguishing between various types of papillary lesions can be problematic. 2,3 Such lesions include intraductal papilloma, intraductal papilloma with atypical ductal hyperplasia, intraductal papilloma with ductal carcinoma in-situ, papillary ductal carcinoma in-situ, intracystic