Uterine serous carcinoma (USC) is the prototype of type II endometrial cancer. Endometrial intraepithelial carcinoma (EIC) is the precursor lesion of USC, Rarely, USC and EIC may arise within and be largely confined to otherwise benign endometrial polyps. This report describes 5 such cases. The patients ranged in age from 67 to 89 years, with a mean age of 75 years. In 2 of the cases there was a history of tamoxifen therapy. In 2 cases USC or EIC was confined to the endometrial polyp, and in 3 cases there was focal involvement of nonpolypoid endometrium. In 1 case there was a single small focus of extrauterine tumor within an ovarian vascular channel. In 2 cases the invasive tumor within the polyp also contained areas of endometrioid adenocarcinoma, and in 2 cases there was a component of clear cell carcinoma. In all cases USC and EIC were strongly reactive for p53 and showed a high proliferation index with MIB1. Two cases were negative with estrogen receptor, and 3 cases exhibited positive staining. The cases reported herein show that USC and EIC may rarely arise in benign endometrial polyps and that extrauterine involvement may be present without myometrial infiltration. Because 2 of the patients had been taking tamoxifen, this raises the possibility of an association between tamoxifen and the development of USC and EIC in the endometrial polyps that are characteristic of this medication.