This study concerns the effects of anti-oestrogens which from now on are an integral part of back-up treatment for cancer of the breast. Seven cases where fibroids led to the need for laparotomy occurred in menopausal women who had been treated with tamoxifen. This has led us to discuss the paradoxical effect of this molecule, which is said to be anti-oestrogenic. This ambivalence is confirmed by studying the literature. These cases raise two questions which we try to answer: 1. Is the action of tamoxifen really anti-oestrogenic? 2. What management should be carried out to lessen the likelihood of tamoxifen treatment leading to laparotomy?