Fluorescence in situ hybridization studies using non-breakpoint DNA probes were performed to detect the X;18 translocation on 4-microm sections of synovial sarcoma from paraffin blocks. This was done by using commercially available, large target unique sequence DNA probes for regions of the X chromosome short-arm and the 18 chromosome long-arm together with centromere probes for the alternate chromosomes. We determined that such probe combinations could detect the presence of the diagnostic X;18 translocation in interphase cells. Spatial association of dual color signals from the X centromere and the 18 unique sequence probe, as well as between an 18 centromere and the X unique sequence probe, was seen in a significantly higher percentage of synovial sarcoma cells (81.1% +/- 7.7%, confidence interval 95%) than in control nonsynovial soft tissue sarcomas (14.7% +/- 8.3%) and control peripheral blood lymphocytes (5.6% +/- 0.6%). The observed spatial association supports the use of this strategy to detect the X;18 translocation in synovial sarcoma and suggests that this technique could be applied in the diagnosis of other types of tumors with characteristic translocations when histopathological findings are inconclusive. This study is the first report describing the use of nonbreakpoint unique sequence probes for detecting translocations in tumors on paraffin-embedded slides.