The authors report a case of an 81-year-old woman affected by a synovial cyst arising from the left facet joint of L3. The patient presented with severe pain and claudicatio spinalis. Diagnosis was based upon standard x-rays and MRI; the patient was operated on and the histology of the lesion confirmed the pre-operative diagnosis. On MRI images the lesion appeared as a roundish extradural mass arising from the left facet joint of L3, compressing the cauda equina posteriorly, and it appeared with an increased heterogeneous signal, consistent with a high quantity of fluid and hemosiderin within the cyst. A communication between the lesion and the joint space was evident on the MRI sagittal reconstruction. Grossly the cut surface of the mass had a cystic aspect, with some hemorrhagic areas between whitish myxoid tissue. Histologically the lesion was formed by fibrous connective tissue covered by synovium; focal inflammation and hemosiderin-laden macrophages were also present in some areas. At one year follow-up the patient is symptom-free.