We describe a case of primary pulmonary synovial sarcoma arising in the lung of a 35-year-old woman.The tumor was a single mass and had a well-defined pleural-based with homogeneous density in the edge of the middle lobe,with no calcification and a fat component on plain CT scan.On the contrast-enhanced CT scan,the tumor showed moderately enhancement and circular enhancement with no enlargement of mediastinal,hilar lymph nodes.Skeleton single photon emission CT(ECT) was unremarkable.Gross and histological findings by postoperative pathology showed the tumor had pseudo-capsule and was well-circumscribed with foci of necrosis,hemorrhage and cystic change,under microscope tumor was typically mitotically active,spindle cells growing in intersecting fascicles or in solid sheets with epithelial differentiation.The immunohistochemical examination of CD99,Vim,PCK and EMA were positive.The tumor recurred with multiple pleural nodules and mass at 5 months after surgery,chest CT revealed the tumor was in the ipsilateral costophrenic angle for chest pain of the patient.After dynamic contrast-enhanced,the solid parts and the capsule of the lesion showed progressive enhancement and the liquefied area of the lesion without enhancement.This case indicates that a primary pulmonary synovial sarcoma is extremely rare and malignant,its prognosis is poor.There are some relatively specific imaging findings. We describe a case of primary pulmonary synovial sarcoma arising in the lung of a 35-year-old woman. The tumor was a single mass and had a well-defined pleural-based with homogeneous density in the edge of the middle lobe, with no calcification and a fat component on plain CT scan. The contrast-enhanced CT scan, the tumor showed moderately enhancement and circular enhancement with no enlargement of mediastinal, hilar lymph nodes. skeletal single photon emission CT (ECT) was unremarkable. Gross and histological findings by postoperative pathology showed the tumor had pseudo-capsule and was well-circumscribed with foci of necrosis, hemorrhage and cystic change, under microscope tumor was typically mitotically active, spindle cells growing in intersecting fascicles or in solid sheets with epithelial differentiation. The immunohistochemical examination of CD99, Vim, PCK and EMA were positive. The tumor recurred with multiple pleural nodules and mass at 5 months after surgery, chest CT revealed the tumo r was in the ipsilateral costophrenic angle for chest pain of the patient. After dynamic contrast-enhanced, the solid parts and the capsule of the lesion showed progressive enhancement and the liquefied area of ​​the lesion without enhancement. This case indicates that a primary pulmonary synovial sarcoma is extremely rare and malignant, its prognosis is poor. Here are some more specific imaging findings.