The authors reported complete regression of a large hemorrhagic lumbar synovial cyst following posterior spinal fusion without direct cystic resection. A 64-year-old woman suffered from sudden onset of the left buttock pain radiating to the left leg after waking up in the morning following the previous history of a minor accident 2 months ago. Magnetic resonance imaging (MRI) of the lumbosacral spine showed a large extradural round mass originating from the left facet joint at the level of L3-L4. The mass was hyperintense on T1-weighted images and hypointense on T2-weighted images, probably compatible with hemorrhagic joint-related cyst. Surgical treatment was chosen for her because of persistent left radicular pain with no responding to medications. The patient underwent decompressive laminectomy, subtotal facetectomy, instrumented fusion, and only tissue biopsy due to severe adherence of the mass and dura. Histopathological examination was consistent with a hemorrhagic synovial cyst. The radicular pain completely disappeared after the surgery. Follow-up MRI of the lumbosacral spine obtained 6 months after the surgery demonstrated complete resolution of the hemorrhagic cyst. Complete resolution of hemorrhagic synovial cyst seems to correlate with subtotal facetectomy, probably resulting in leakage of cyst content and subsequent resorption of the cyst wall. In addition, hematoma within the synovial cyst may resolve spontaneously over time.