In recent years, the extensive usage of tamoxifen in patients with breast cancer has led us to acquire a great deal of knowledge of its effects on various organs in the gynecological system, especially the effect on the endometrium and the increased risk of endometrial cancer. Information on the possible potential carcinogenic effect of tamoxifen on the ovary, however, has been limited, mainly because of the overall low incidence of ovarian carcinoma. In addition, there is the confounding variable posed by the diverse here-ditary breast and ovarian cancer syndromes which tend to occur in a younger age group. Here, we present a case of a postmenopausal woman who was treated for six years with tamoxifen for breast cancer before being diagnosed with endometrioid carcinoma of the ovary. We believe that given the age of the patient and the duration of tamoxifen use, the occurrence of ovarian endometrioid carcinoma may be associated with long-term tamoxifen use.