Three facts are strongly supported by clinical and experimental evidence. First, postmenopausal women produce estrogen, occasionally in sufficient quantities to be associated with uterine bleeding and adenocarcinoma. The estrogen is produced in extraglandular sites and is not secreted by the ovaries. Second, the estrogen produced by these women is biologically active. Third, many of the biologic effects of estrogen are antagonized by progesterone. None of these observations proves that estrogen causes endometrial cancer but each can be added to the growing body of evidence that suggests a casual role for estrogen.