Synovial sarcoma is a high-grade soft tissue sarcoma that can be challenging to diagnose on the basis of histology alone. It is defined by a characteristic translocation t(X;18) that produces the fusion oncogene SYT-SSX. The current diagnostic gold standard for synovial sarcoma is the demonstration of the translocation by fluorescence in situ hybridization, reverse transcriptase polymerase chain reaction, or cytogenetics, in an appropriate histologic context. TLE1 encodes a transcriptional corepressor that is overexpressed in synovial sarcomas. Gene and tissue microarray studies have identified TLE1 as an excellent bio-marker for distinguishing the synovial sarcoma from other soft tissue malignancies. We prospectively evaluated incoming soft tissue tumor cases where the histology and clinical setting made synovial sarcoma a real consideration in the differential diagnosis. TLE1, Bcl2, epithelial membrane antigen, and cytokeratin expression were assessed using commercially available antibodies. TLE1 gave intense, diffuse nuclear staining in 35 of 35 molecularly confirmed synovial sarcoma cases, and was rare to absent in the 73 other soft tissue tumors examined (positive staining was found only in 1 of 43 malignant peripheral nerve sheath tumors, the 1 tested fibrosarcoma, and 1 pleomorphic sarcoma). TLE1 was more sensitive and specific for synovial sarcoma than other currently available immunohistochemical markers including Bcl2, epithelial membrane antigen and cytokeratins, and had a positive predictive value of 92% and a negative predictive value of 100% in this clinical setting. Our findings confirm, in a prospective diagnostic context, that TLE1 is more sensitive and specific for synovial sarcoma than any other currently available immunohistochemical stains, and in some cases may preclude the need for molecular testing.