Mounting cytogenetic evidence indicates that synovial sarcomas, regardless of histological conformation, share the specific reciprocal chromosomal translocation t(X;18)(p11.2;q11.2). Application of dual-colour fluorescence in situ hybridization (FISH) on interphase nuclei isolated from archival paraffin-embedded material to identify the specific translocation is of diagnostic importance for pathological practice and retrospective study. Five cases of well-characterized biphasic synovial sarcomas, two monophasic fibrous synovial sarcomas, one embryonal rhabdomyosarcoma, one fibrosarcoma, and one malignant peripheral nerve sheath tumour were analysed. To visualize the translocated chromosomal fragments and their topographic relationships with centromeres of chromosomes X and 18, nuclei from each case were hybridized concurrently with chromosome X centromeric and chromosome 18 painting probes, and chromosome 18 centromeric and chromosome X painting probes, respectively. Six out of seven synovial sarcomas showed chromosomal alterations consistent with t(X;18). One biphasic synovial sarcoma had trisomy 18 and lacked the chromosomal translocation t(X;18). The other three spindle cell sarcomas and the normal control tissues showed the normal numerical and structural composition for chromosomes X and 18. It is indicated from the present study that when histological differential diagnosis is difficult, FISH would be a crucial aid in detecting a known specific chromosomal alteration and that dual-colour FISH is an efficient stable diagnostic tool for pathological research and daily diagnosis. The results also suggest that rare synovial sarcomas may lack the chromosomal translocation t(X;18). 1998 John Wiley & Sons, Ltd.