METHODS A case report. OBJECTIVE To illustrate a rare case of synovial sarcoma arising within a peripheral nerve. BACKGROUND A synovial sarcoma arising within a peripheral nerve is very unusual. Only five cases of primary synovial sarcoma within a peripheral nerve have been reported. This is the first case with involvement of the nerve root. The authors diagnosed the tumor arising within the S1 nerve root as synovial sarcoma using cytogenetic analysis that detected the chimeric SYT/SSX gene. METHODS In addition to the immunohistochemical study, a reverse transcription-polymerase chain reaction (RT-PCR) assay was conducted for the SYT-SS10 fusion gene using archival formalin-fixed paraffin-embedded tumor specimens. RESULTS Computed tomography scan, magnetic resonance imaging performed before surgery, and the intraoperative findings showed that the tumor was embedded within the S1 nerve root. Although the histologic findings were suggestive of a malignant peripheral nerve sheath tumor, the results of the cytologic study confirmed its diagnosis of synovial sarcoma. CONCLUSIONS Primary intraneural synovial sarcoma, although rare, must be distinguished from malignant peripheral nerve sheath tumor. The molecular assay of the detection of the SYT/SSX fusion gene is useful to make a definite diagnosis of monophasic synovial sarcoma.