A 57-year-old female patient with synovial sarcoma in her right foot had a chromosome abnormality defined as translocation (X;18). The tumour was located in the subcutis, and histological investigation showed monophasic proliferation of oval to spindle-shaped cells with a fascicular arrangement lacking an epithelial component. Immunostaining disclosed no cytokeratin or epithelial membrane antigen in tumour cells. Karyotypic analysis revealed translocation (X;18) in addition to other nonspecific aberrations. Fluorescence in situ hybridization was carried out on paraffin-embedded tissue, using DNA probes for the centromeres of chromosomes X and 18 with whole chromosome painting probes for X and 18. The free nuclei showed two signals at a rate of 83-85% with the X and 18 centromeric probes, in contrast to three signals at a rate of 68-70% with the X and 18 painting probes.