BACKGROUND: The t(X;18) translocation is a specific marker of synovial sarcomas (SS). Detection of SYT-SSX transcripts by polymerase chain reaction (PCR) was tested on preselected specimens of well-established histologic types, but to our knowledge, the diagnostic utility of molecular assays on a series of potential SS in comparison with conventional tools has never been reported. METHODS: Two hundred four consecutive cases of potential SS submitted for a second opinion were studied prospectively. On the basis of clinical context, histologic aspect, and immunohistochemical profile, the tumors were divided into three categories: 1) diagnosis of SS certain, when the only possible diagnosis was SS; 2) diagnosis of SS probable, when SS was the first diagnosis contemplated, but a differential diagnostic issue was raised by other tumors; 3) diagnosis of SS possible, when the diagnosis of SS was not the first diagnosis considered. Detection of SYT-SSX transcripts was performed using real-time PCR from fixed, embedded tissue as a systematic test. RESULTS: Sufficient RNA samples were recovered for PCR from 177 specimens (87%). One hundred four specimens (51%) were positive for SYT-SSX transcripts. Tumor sites of SS included the extremities (n = 57), lung (n = 13), trunk wall (n = 12), head and neck (n = 6), and other sites (n = 16). There were 61 monophasic, 22 poorly differentiated, 17 biphasic, and 4 predominantly epithelial SS. For 58 tumor specimens (29%), diagnosis of SS was certain before molecular testing; 49 (84.5%) of these 58 contained SYT-SSX transcripts. For 39 tumor specimens (19%), diagnosis of SS was probable; 29 (74.4%) of these 39 contained SYT-SSX transcripts. For 107 tumor specimens (52%), diagnosis of SS was only possible and strongly challenged by another histologic type. The issue consisted mainly of making the distinction between an SS and a poorly differentiated spindle cell sarcoma (n = 49), a poorly differentiated round cell sarcoma (n = 34), a carcinoma (n = 11), a myoepithelioma (n = 8), or an epithelioid fibrosarcoma (n = 5).Twenty-six tumor specimens (24.3%) contained SYT-SSX transcripts-10, 7, 5, 3, and 1 in the spindle cell tumor, round cell tumor, carcinomalike tumor, myoepitheliomalike tumor, and epithelioid-fibrosarcoma-like tumor categories, respectively. CONCLUSIONS: Molecular testing was not required if the diagnosis of SS was certain or probable on the basis of clinical, histologic, and immunohistochemical evaluation. However, it proved to be very helpful or necessary when the diagnosis of SS was only possible and was challenged by other tumor types, mainly other spindle cell sarcomas, round cell sarcomas, carcinomas, myoepitheliomas, and epithelioid fibrosarcomas