Despite the major public health impact of leiomyomas, little is known about their cause. Until recently, the steroid hormones estrogen and progesterone were considered the most important regulators of leiomyoma growth. There is abundant evidence that estrogen promotes fibroid growth including the clinical observations that fibroids grow in the presence of high levels of estrogen, such as during the reproductive years, and that they regress in the presence of low levels of estrogen, such as following menopause or during gonadotropin releasing hormone (GnRH) agonist therapy.