GATA3 plays a role in cell proliferation and differentiation in many tissues, including breast, and it has been suggested that GATA3 expression correlates with ER expression. However, little is known on GATA3 expression in various subtypes of breast carcinoma, its utilization in cytology, and on how GATA3 performs in comparison with GCDFP-15 and mammaglobin. Eighty-four histology cases of breast carcinoma of various subtypes, including 28 triple-negative breast carcinomas, along with 20 cytology cases of metastatic breast carcinoma and 12 cytology cases of ER-positive metastatic gynecologic malignancies, were stained for GATA3, GCDFP-15, and mammaglobin. In non-triple-negative breast carcinomas (n=56), GATA3 showed 100% sensitivity, higher than GCDFP-15 (42.8%; P<0.0001) and mammaglobin (58.9%; P<0.0001), whereas staining patterns were similar for all the histologic subtypes examined. Staining scores were determined by multiplying the percentage of cancer cells staining with an intensity score of 1+, 2+, or 3+ (range, 0 to 300). In non-triple-negative carcinomas, GATA3 showed a mean score of 273.7, higher than GCDFP-15 (107.5; P<0.0001) and mammaglobin (147.7; P<0.0001). In triple-negative breast carcinomas (n=28), GATA3 showed a sensitivity of 60.7%, greater than GCDFP-15 (17.9%; P=0.0022) and mammaglobin (7.1%; P<0.0001). These results were consistent irrespective of the subtype examined. In breast carcinoma cytology cases, 100% stained with GATA3, higher than GCDFP-15 (20%; P<0.0001) and mammaglobin (45%; P<0.0001). None of the metastatic endometrial or ovarian carcinomas were positive for GATA3. Although GATA3 expression correlates with ER expression in breast, no correlation is observed in gynecologic malignancies. Thus, in working up ER-positive metastatic malignancies GATA3 demonstrates specificity for breast.