Abnormal proliferation of smooth muscle cells in pulmonary lymphangioleiomyomatosis (LAM) is thought to be influenced by estrogen and progesterone. However, the results of previous studies using cytosolic methods to measure estrogen and progesterone receptor content in lung tissue from these patients have been inconsistent. We used immunohistochemica) methods to study the tissue distribution of estrogen and progesterone receptors in LAM as well as in smooth muscle of several other organs, including histologically normal lung, colon, bladder, prostate, uterus, and uterine leiomyomas. Progesterone receptor was expressed strongly and estrogen receptor more weakly by the abnormal myoid cells of LAM. Hormone receptors were absent from all other constituents of lung tissue in our patient. These findings were similar to those in histologically normal myometrium and uterine leiomyomas. Although we found focal labeling of prostatic stromal cells with anti-progesterone receptor, no other smooth muscle tissue expressed either estrogen or progesterone receptor. We conclude that LAM is an abnormal proliferation of smooth muscle cells that express both estrogen and progesterone receptors.