"Benign metastasizing leiomyoma" is the terminology used to describe a controversial entity characterized by a proliferation of bland-appearing smooth muscle in lung or abdominopelvic lymph nodes. In this report, we describe 5 cases of pulmonary-based smooth muscle tumors that are clinically and histologically consistent with this entity, and in which we identified consistent chromosomal aberrations (19q and 22q terminal deletion in all cases). This cytogenetic profile is found in approximately 3% of uterine leiomyoma, but has not been described in other types of benign or malignant neoplasia. These findings suggest that the nodular pulmonary smooth muscle proliferations termed "benign metastasizing leiomyoma," are a genetically distinct entity, which likely originate from a biologically distinctive subset of uterine leiomyoma.