Objective. To evaluate and compare HER2/neu protein overexpression and gene amplification in uterine serous papillary endometrial cancer (USPC). Study design. Immunohistochemical (IHC) and fluorescent in situ hybridization (FISH) assays were used to analyze and compare HER2/neu protein expression and gene amplification, respectively, in paraffin blocks from 26 women harboring stage IA to IV USPC treated at the University of Arkansas for Medical Sciences from 1997 to 2004. Chromosome 17 polysomy status by FISH was also assessed in all specimens. Results. Moderate-to-strong expression of HER2/neu protein was noted in 16 (62% ) of 26 USPC samples evaluated, with 7 (27% ) samples showing moderate staining (2? ) and 9 (35% ) showing strong staining (3? ) for HER2/neu. Amplification of the ERBB2 gene by FISH was observed in 11 of the 26 (42% ) cases. Protein overexpression and gene amplification were found to correlate in 100% (9 of 9) of the 3? positive tumors and 2 out of 7 (29% ) of the 2? positive tumors. Heterogeneity was noted in 3 cases in the amplification of the HER2/neu gene within the same tumor samples with pockets of amplified tumor cells amidst nonamplified tumor cells. None of the 10 USPC cases scored by IHC as 0 or 1? was found positive for ERBB2 amplification by FISH. Conclusions. Amplification of the HER2/neu oncogene represents a common finding in USPC. FISH analysis should be used for confirmation of gene amplification in USPC showing 2? expression of HER2/neu. Prior screening and selection of appropriate immunohistochemistry-positive areas may be beneficial in the selection of some USPC patients undergoing FISH analysis. Objective. To evaluate and compare HER2 / neu protein overexpression and gene amplification in uterine serous papillary endometrial cancer (USPC). Study design. Immunohistochemical (IHC) and fluorescent in situ hybridization (FISH) assays were used to analyze and compare HER2 / neu protein expression and gene amplification, respectively, in paraffin blocks from 26 women harboring stage IA to IV USPC treated at the University of Arkansas for Medical Sciences from 1997 to 2004. Chromosome 17 polysomy status by FISH was also assessed in all specimens. Results. Moderate- (27%) samples showing moderate staining (2?) and 9 (35%) showing strong staining (3?) for HER2 / neu. Amplification of the ERBB2 gene by FISH was observed in 11 of the 26 (42%) cases. Protein overexpression and gene amplification were found to correlate in 100% (9 of 9) of the 3? positive tumors and 2 out of 7 (29%) of the 2 ? positive tumors. Heterogeneity was noted in 3 cases in the amplification of the HER2 / neu gene within the same tumor samples with pockets of amplified tumor cells amidst nonamplified tumor cells. None of the 10 USPC cases scored by IHC as 0 or 1? was found positive for ERBB2 amplification by FISH. Conclusions. Amplification of the HER2 / neu oncogene represents a common finding in USPC. FISH analysis should be used for confirmation of gene amplification in US PC showing 2? expression of HER2 / neu. Prior screening and selection of appropriate immunohistochemistry-positive areas may be beneficial in the selection of some USPC patients undergoing FISH analysis.