Adjuvant tamoxifen therapy for breast cancer patients has been found to be associated with various endometrial pathologic conditions, including endometrial cancer. This preliminary case control study evaluated whether prolonged and continuous exposure to tamoxifen in the menopause may aggravate existing endometrial pathologies. Two vaginal ultrasound evaluations of endometrial thickness and histologic findings of two endometrial biopsies, performed 18 months apart, were evaluated for 25 asymptomatic, postmenopausal breast cancer patients who were continuously treated with tamoxifen. In the first endometrial biopsy, 4 patients (16.0%) were found to have endometrial pathologies: 2 patients had proliferative endometrium, 1 had hyperplastic endometrium, and 1 had an endometrial polyp. In the second endometrial biopsy, none of these patients showed any endometrial pathologies. Another patient (4.0%) with no endometrial pathology in the first visit had endometrial hyperplasia in the second visit. None of the patients developed endometrial cancer, and generally there was no increase in ultrasonographically-measured endometrial widths. The results of this preliminary study may suggest that there is no increased risk of development of endometrial pathologies during an additional 18 months of continuous tamoxifen therapy nor is there aggravation of already existing endometrial pathologies.