To evaluate possible predicting factors for endometrial cancer in postmenopausal breast cancer patients with tamoxifen treatment, we compared various clinical features between 12 postmenopausal breast cancer tamoxifen-treated patients with endometrial cancer and a control group of 261 otherwise similar patients without this endometrial pathology. These comparisons were based on a long-term prospective follow-up. Several clinical factors such as longer duration of breast disease, older patient age, lower frequency of chemotherapy administration, and higher frequency of postmenopausal bleeding were found among the tamoxifen-treated patients with endometrial cancers, and were significantly different when compared to the control group. Only eight (66.7%) had postmenopausal bleeding, and a preoperative diagnosis of endometrial cancer was made in only six (50.0%). When considering postmenopausal bleeding as a marker for endometrial cancer in the study patients, sensitivity was 67% and specificity was 98%.