HOX genes are essential regulators of development in all multicellular organisms, including humans. We have previously shown that HOXA10 is expressed in the developing uterus and later in the adult human endometrium. HOX genes regulate endometrial development in response to sex steroids. Here, we demonstrate that HOXA10 is expressed in the myometrium as well. In situ hybridization reveals abundant HOXA10 expression, and Northern analysis demonstrates differential HOX gene expression in the myometrium throughout the menstrual cycle. HOXA10 expression decreases in the midsecretory phase, coinciding with high serum progesterone levels. Treatment of primary myometrial cell cultures with progesterone decreases HOXA10 expression in vitro-paralleling the expression seen in vivo. Despite the presence of progesterone receptors in the endometrium and myometrium, HOXA10 is differentially regulated in each tissue by progesterone. HOXA10 expression is induced in the stroma and decreased in the myometrium by progesterone. The differential tissue-specific response of this gene in response to progesterone is likely mediated by sex steroid receptor coactivators or corepressors. Decreased myometrial expression of developmental regulatory genes such as HOXA10 in the nonpregnant uterus may dedifferentiate the myometrium and allow growth in preparation for pregnancy.