Low-grade endometrial stromal sarcoma (LG-ESS) is a rare uterine neoplasm. Computed tomography (CT) revealed the presence of multiple small bilateral pulmonary nodules in a 58-year-old woman 1 year after surgery for LG-ESS; the clinical diagnosis was pulmonary metastasis. Hormone therapy with progesterone was initiated, after which most of the solid nodules disappeared and some transformed into cystic lesions. Seven years after hormone therapy, the patient experienced repeated pneumothorax. The cause of the pneumothorax was perforation of a metastatic focus within the wall of a small subpleural cyst that was not evident on CT images.