Tamoxifen is currently the most widely used drug for the treatment of breast cancer, but there now exists considerable evidence that tamoxifen can also induce endometrial hyperplasia in pre menopausal women. We have used PCR differential display on primary human endometrial isolates in an attempt to identify genes induced by tamoxifen but not estrogen. Eight such differentially expressed bands were cloned and sequenced, one of which was found to be the peptide adrenomedullin. We have shown that adrenomedullin is a novel growth factor for endothelial cells and is angiogenic in vivo in the chick chorioallantoic membrane assay. Immunohistochemical analysis of endometrial sections have shown that while macrophages in the endometrium express adrenomedullin at a low level, endometrial macrophages of women receiving tamoxifen strongly express adrenomedullin (P= 0.008). We postulate that endometrial induction of the angiogenic factor adrenomedullin by tamoxifen is part of the mechanism by which tamoxifen results in endometrial hyperplasia.