Core needle biopsies of breast carcinomas provide diagnostic, prognostic, and predictive information before neoadjuvant therapy. Possible intratumoral heterogeneity of biomarker expression questions the validity of core needle biopsy interpretation in small biopsy specimens. Using tissue microarray (TMA) technology, we studied intratumoral heterogeneity of 7 immunomarkers. Five TMAs were constructed from 44 breast carcinomas and 5 normal breast tissues, each represented by 1-mm cores in triplicate from each of 3 foci. TMAs were immunostained for monoclonal estrogen receptor (ER), monoclonal progesterone receptor (PR), polyclonal human epidermal growth factor receptor 2 (HER2), monoclonal E-cadherin (E-cad), monoclonal epidermal growth factor receptor (EGFR), monoclonal p53, and monoclonal MIB-1. Expression was quantified visually by light microscopy and by image cytometry as intensity, percentage of cells positive, and score. Using intraclass correlation coefficient, heterogeneity in the expression of the immunomarkers within subjects was compared with the overall variance. Intratumoral heterogeneity was seen with 5 immunomarkers: ER, PR, HER2, p53, and MIB-1. E-cad and EGFR failed to show intratumoral heterogeneity. Intratumoral heterogeneity in ER, PR, HER2, p53, and MIB-1 indicates their problematic interpretation in small biopsy specimens as indicative of the status of the entire tumor. A negative result does not exclude the expression of these markers in the remainder of the tumor. E-cad (positive in ductal carcinomas) and EGFR lacked heterogeneity.