In recent years, the association between breast and endometrial cancer has been the subject of many studies. The present report describes four cases of this association in which tamoxifen had been administered to all of the patients. Data have been published regarding the possibility that tamoxifen may be responsible for the subsequent development of carcinoma of the corpus uteri in these patients. The authors intend to carry out a case-control study on patients treated with tamoxifen for breast carcinoma to reveal the possible presence of endometrial carcinoma.