This case describes a 46-year-old woman with local pelvic and perineal pain, persisting for 2 years at presentation. The pain worsened during the night and morning and was alleviated during daily activities. Low back pain was associated with morning stiffness lasting longer than 2 h. Sometimes, she felt pain and numbness along her left S1 dermatome, without overt bladder or bowel incontinence. Lasegue's sign was negative. Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) were elevated (35 mm/h and 9.4, respectively) and Mennel's sign was present on both sides, indicating possible inflammation of the sacroiliac joints. However, radiographs of the lumbosacral spine and sacroiliac joints were normal. Magnetic resonance imaging (MRI) revealed a large spinal meningeal cyst in the sacrum (60 × 37 × 22 mm) consisting of multiple perineural cysts. The cyst eroded the surrounding sacral bone structures, narrowed several sacral foramina, and compressed neighboring nerve fibers. MRI findings on sacroiliac and hip joints were normal.