Uterine fibroids are the commonest of pelvic neoplasms and therefore should command the interest of all gynecologists. What is the source of these tumors is a question that is asked daily and one which can be answered only in generalities. In three former contributions (1, 2, 3) excessive stimulation of the myometrium by the ovarian follicular hormone has been suggested as a cause. That the action of this hormone is not specific for the endometrium, as demonstrated by the endometrial changes during the normal menstrual cycle, but that it affects the genital tract as a whole, is easily proved (4). When the action on the endometrium is abnormal, with resulting endometrial hyperplasia, there is an equally abnormal action upon the myometrium, which, if it is prolonged sufficiently, results in cellular metaplasia of the uterine muscle cell or cells, with the subsequent development of uterine fibroids.