Synovial sarcoma is the most common nonrhabdomyosarcomatous soft-tissue sarcoma in children and young adults. It is characterized by the common t(X;18)(p11.2;q11.2) that results in the fusion of SYT on chromosome 18 to one of two closely related and adjacent genes on the X chromosome, SSX1 or SSX2. Here we describe a poorly differentiated, monophasic synovial sarcoma in a 17-year-old adolescent boy. Hyperdiploidy, a t(X;18)(q13;q11), and other structural abnormalities were detected by conventional cytogenetic analysis. Fluorescence in situ hybridization with the PAC probe RP3-519N18, which is specific for the Xp11 region, resulted in a signal on the der(Xq), a finding consistent with a pericentric inversion of the X chromosome that resulted in a t(X;18)(p11.2;q11.2)inv(X)(p11.2q13). Real-time polymerase chain reaction using primer sets specific for SYT-SSX1 and SYT-SSX2 confirmed the presence of an SYT-SSX1 fusion transcript. Our finding of this unique and complex translocation in synovial sarcoma demonstrates the utility of molecular methods in confirming the diagnosis of synovial sarcoma.