GATA3 is a transcription factor that is involved in the embryonic development of the parathyroid glands and in adult parathyroid cell proliferation. The aim of this study is to investigate the expression of GATA3 in parathyroid tumors and to determine whether it could be used as an immunohistochemical marker for parathyroid differentiation in tumors. Immunoreactivity for GATA3 was nuclear and was demonstrated in all 10 hyperplastic parathyroid glands, 22 parathyroid adenomas, and 6 parathyroid carcinomas; whereas, all thyroid tumors, renal cell carcinomas, thymic epithelial tumors, and carcinoid tumors investigated for comparison purposes were negative for this marker. It is concluded that GATA3 is a very sensitive and relatively specific immunohistochemical marker for parathyroid differentiation that can assist in the differential diagnosis of parathyroid tumors.