Cytogenetic analysis of a synovial sarcoma of the base of the tongue showed a reciprocal translocation involving chromosomes X and 18 [t(X;18)(p11.2;q11.2)]. This is a translocation recently reported to be characteristic of synovial sarcomas of the extremities. The histogenesis of synovial sarcoma is controversial. Our discovery of this translocation in an oral synovial sarcoma confirms the unity of origin of this neoplasm with the far more common synovial sarcomas of the extremity. Karyotypic analysis may prove useful in confirming the diagnosis of this uncommon neoplasm.