Hormone receptor status determination for breast cancer is an important part of pathologists' daily sign outs and many retrospective and prospective studies. In this study, we compared the estrogen receptor (ER) expression tested on tissue microarray (TMA) sections to those tested on whole sections (WS) to find out concordance and frequency of intratumoral heterogeneity. Five and one-half percent of all tumors showed discrepancy in ER expression between TMA and WS. The rate of discrepancy was lower with increasing number of cores from individual cases. In 1.4% of cases, there was discrepancy in ER expression between cores in which >1 core was available on TMA section. We concluded that TMA can be used to determine ER status with good concordance to ER determination done on WS. Whenever the size of the tumor in the block allows it, more cores should be taken to construct TMAs.