Synovial sarcoma is characterized cytogenetically by translocation t(X;18)(p11.2;q11.2). In this study, 28 cases that had been diagnosed initially as synovial sarcoma, including 2 fibrosarcomas, and 1 leiomyosarcoma were collected and examined for translocation t(X;18) on paraffin-embedded tissues by fluorescence in situ hybridization (FISH). Of the synovial sarcomas, 25 showed findings consistent with translocation t(X;18) with an additional copy signal for the total probe of X and 18 chromosomes. The other three cases, as well as the two fibrosarcomas and the leiomyosarcoma, did not show this translocation. One (case 26) of three negative cases was diagnosed finally as leiomyosarcoma and another (case 27) as malignant peripheral nerve sheath tumor from histological and immunohistochemical analysis. Thus, in all, 25 (96%) of 26 synovial sarcomas showed findings consistent with translocation t(X;18). In summary, translocation t(X;18) is a chromosomal aberration specific for synovial sarcoma. The fluorescence in situ hybridization technique can be used even on cells from paraffin-embedded tissues, and is a useful diagnostic aid for synovial sarcoma.