The authors report an unusual spindle cell sarcoma that arose in the lung of a 12-year-old girl. This tumor had histologic, immunophenotypic, and ultrastructural features consistent with monophasic fibrous synovial sarcoma. These features included a growth pattern of densely packed spindle cells in irregularly intersecting, broad fascicles, diffuse vimentin immunoreactivity, and focal expression of epithelial membrane antigen and S100 protein. This diagnosis was further supported by cytogenetic studies showing the specific t(X; 18) chromosomal translocation associated with synovial sarcoma. This balanced translocation appears to be an essentially universal characteristic of these sarcomas, regardless of histologic subtype or site of origin. The constellation of morphologic and cytogenetic findings in this case firmly establishes synovial sarcoma as a subtype of pulmonary spindle cell sarcomas. The distinctive features of these neoplasms allow them to be distinguished from a variety of primary and metastatic malignancies in the lung.