We present 25 cases of a primary pulmonary sarcoma bearing histological, immunohistochemical, and ultrastructural features indistinguishable from those of monophasic synovial sarcoma of soft tissue. The patients were 11 men and 14 women between the ages of 16 and 77 years. Clinically, the most common symptoms were chest pain, cough, shortness of breath, and hemoptysis. The lesions involved all lung segments. Grossly, they varied in size from 0.6 to 20 cm and were described as soft to rubbery tumors with areas of necrosis and hemorrhage, some with cystic changes. Two lesions involved the bronchial wall and in one case the tumor was described as encircling the bronchial tree. Histologically, all of the lesions were characterized by an atypical spindle cell proliferation with a solid growth pattern. Areas of myxoid, neural, hemangiopericytic, and epithelial-like growth pattern were observed. Mitoses, necrosis, and hemorrhage were seen in all lesions in varying proportions. Immunohistochemical studies for epithelial membrane antigen (EMA) and keratin showed strong focal positivity in 25 of 25 and 23 of 25 lesions, respectively. Immunohistochemical study for vimentin showed diffuse strong positivity in all lesions. Other immunostains, including desmin, smooth muscle actin, and S-100 protein, were negative. Electron microscopy in three cases showed spindle cells with elongated nuclei containing abundant cytoplasmic rough endoplasmic reticulum and well developed desmosome type intercellular junctions. Follow-up information ranging from 2 to 20 years was obtained in 18 patients. Six patients died of their tumors, whereas four patients died of unrelated causes without evidence of recurrence or metastases. Eight patients were alive with disease (recurrence and/or metastases) from 1 to 7 years after diagnosis. Four patients were alive and well without evidence of recurrence or metastases from 2 to 20 years (mean follow-up, 12.5 years). The present group of lesions appears to constitute a distinctive and as yet previously undescribed primary sarcoma of the lung, which probably represents the visceral counterpart of monophasic synovial sarcoma of soft tissue in a pulmonary location. Because of their distinctive biology these lesions should be distinguished from a variety of primary and metastatic malignancies of the lung.