Uterine papillary serous carcinoma (UPSC) is a highly aggressive variant of endometrial cancer with features similar to high-grade ovarian cancer. Patients tend to be elderly, thin, have a high grade tumor with extensive extrauterine disease at the time of diagnosis. The transmembrane receptor encoded by the HER-2 cellular oncogene is amplified in several types of human carcinomas and provides an attractive therapeutic target. HER-2/neu, the transmembrane receptor encoded by the c-erbB2 gene, is overexpressed by immunohistochemistry in <25% of ovarian cancers and 20-30% of breast cancers, and <10% of endometrial cancer. There are prognostic and therapeutic implications associated with the overexpression of this transmembrane protein. Herceptin, a humanized murine monoclonal antibody directed against the extracellular domain of the HER-2/neu protein, is being used to treat breast cancer that overexpresses HER-2/neu. We reviewed all patients diagnosed with UPSC between 1999-2001. Twenty-six patients were identified, and 19 patients had specimens available for evaluation. We performed immunohistochemical analysis (Herceptest, Dako, Carpinteria, CA) on 19 paraffin embedded blocks of UPSC tumors looking for HER-2/neu over expression. Five out of 19 (26%) stained heavily (3+) for HER-2/neu receptor protein. Four of these five patients had advanced disease at diagnosis. Two of these patients were subsequently treated with Herceptin; one with complete response and one with stable disease based on CT scan and CA-125 findings. Targeting HER-2/neu may be beneficial for a select group of patients with UPSC. We are continuing to evaluate samples for HER-2/neu over expression by fluorescence in situ hybridization (FISH).