Pulmonary metastases of endometrial stromal sarcoma (ESS) are uncommon and can pose diagnostic problems. We reviewed lung specimens from 16 patients with metastatic ESS. Patients were 31-77 years of age at the time of lung biopsy. Uterine ESSs were diagnosed an average of 9.8 years before lung biopsy in 11 patients. Uterine ESSs were originally called smooth muscle tumors in three additional patients. Thirteen patients were evaluated for new pulmonary nodules, seven of whom were asymptomatic. Nodules were multiple in 14 and solitary in four, ranging from 1.0 to 8.0 cm in greatest dimension. One patient died of metastatic disease; 14 were alive and seven of these were without disease (mean follow-up 4.1 years). Diagnostic considerations in 12 consultation cases included ESS, sclerosing hemangioma, carcinoid tumor, lymphangioleiomyomatosis, endometriosis, hemangiopericytoma, and lymphoma. Tumors were well circumscribed and usually solid, composed of plump spindle cells arranged in short fascicles. Two tumors were predominantly cystic. Sex cord-like stromal differentiation was identified in three. Neoplastic cells stained for vimentin (93%), estrogen and progesterone receptor (100%), smooth muscle actin (57%), desmin (50%), and keratin (46%). Metastatic ESS should be included in the differential diagnosis of nonepithelial neoplasms in women.