WT1 diffusely stains most ovarian serous carcinomas; reactivity of uterine papillary serous carcinomas has not been evaluated. We studied WT1 expression in 13 International Federation of Gynecology and Obstetrics stage 1 and 5 stage 3 or 4 uterine papillary serous carcinomas without ovarian metastases and compared their reactivity with the WT1 staining of 30 ovarian serous carcinomas. WT1 reactivity was evaluated with the C19 and 6F-H2 antibody clones. All 18 uterine papillary serous carcinomas were nonreactive for WT1. The nonovarian metastases of the 5 high-stage uterine papillary serous carcinomas also were nonreactive for WT1. In contrast, 29 (97%) of 30 ovarian serous carcinomas were reactive for WT1. WT1 reactivity in an unknown primary serous carcinoma would suggest it is from a nonuterine site. The mechanisms underlying these findings are unknown. They raise the possibility of genetic differences between the 2 morphologically similar neoplasms.