Relapsing spontaneous pneumothorax can be the first manifestation of pulmonary metastases of soft tissues sarcomas. Standard imaging techniques and computed tomography may not be able to detect small malignant cystic lesion or to distinguish between them and benign bollous lesions. We report the case of a 33 year-old male who, in the past, underwent surgical treatment for a synovial sarcoma of the inferior limb. The patient was admitted to our hospital because of right spontaneous pneumothorax; both chest x-ray and CT scan didn't detect any metastatic pulmonary lesion. A few days after the discharge the patient was readmitted because of relapsed pneumothorax; high-definition CT of the chest revealed a pulmonary cystic lesion that was resected thoracoscopically. Histological examination revealed a pulmonary metastases of synovial sarcoma.