Seventy-four cases of so-called "benign metastasizing uterine leiomyomata" are reported in the literature. In these cases, well differentiated, leiomyomatous lung tumors developed, usually after a period of several years. Histologically, these tumors appear to be benign. We report on five more such cases in which we investigated the contents of estrogen and progesterone receptors in the pulmonary tumors by immunohistological procedures. All the lung tumors exhibited a high content of progesterone receptors, and in 4 out of the 5 cases a high estrogen receptor content was also found. Modern immunohistological techniques permit the investigation of routinely fixed tissue blocks, and it is thus recommended that the contents of these hormone receptors should be determined in well differentiated, leiomyomatous lung tumors from women. This would both provide information on the pathogenesis of these tumors and establish a basis for possible later institution of hormone treatment. It is likely that the majority of these lung tumors are in fact metastases of extremely well differentiated leiomyosarcomas of the uterus. The possibility that lung tumors of this type may constitute a small group that develop in situ as hormone-sensitive proliferations cannot, however, be fully excluded.