Between September 1, 1989 and October 31, 1994, 173 postmenopausal breast cancer women on tamoxifen treatment were followed up in the authors' institutions. During this period, 14 (8.1%) underwent total abdominal hysterectomy and bilateral salpingo-oophorectomy for various indications. Eight (57.1%) were found to have adenomyosis, of whom one had a large fundal adenomyotic lump and the other seven patients had two to four small microscopic foci of adenomyosis. In this study, the rate of adenomyosis described among those postmenopausal breast cancer patients treated with tamoxifen is nearly three to four times higher than the rate reported in the literature for pre- and postmenopausal women. There is no previous reported increased incidence of adenomyosis in postmenopausal breast cancer patients treated with tamoxifen. Thus, it is suggested that the prolonged and unopposed estrogen-like stimulation by tamoxifen may play a causal role rather than be a casual factor in the development of this pathologic entity.