A 47-year-old woman presented to the Interdisciplinary Spine Center complaining of pain in her abdomen and left leg. She had no splenomegaly, hepatomegaly, or any tenderness on palpation of the abdomen in the lower left side. There was also no pain on percussion of the thoracolumbar and lumbosacral spine. On auscultation, a normal active bowel sound was heard. Superficial sensory examination: pins and needles, light touch, and proprioception were intact. Muscle strength, muscle tone, and reflexes of the upper and lower extremities were normal. Hoffman and Babinski’s disease were negative. There were no motor or sensory abnormalities. Rectal palpation showed normal anal tone and no significant masses. She exhibited a normal gait. Sacroiliac joint provocative movements, including Gaenslen’s test, Fabere’s test, sacroiliac joint compression test, shear test, iliac gap test, and Yorman’s test were negative. Lumbar discogenic provocative maneuvers, including pelvic rock and sustained hip flexion, were negative. MRI of the thoracic spine revealed no abnormalities. Lumbosacral MRI showed a large perineural cyst eroding the sacrum and extending into the retroperitoneum. No other significant abnormalities were found. Comments: 1. Although sacral cysts causing abdominal pain are relatively rare, we need to be more careful in the differential diagnosis in clinical work to avoid missed diagnosis and misdiagnosis. 2.At present, clinically, the case of huge sacral cysts protruding into the pelvis is relatively rare. The literature reports that only about 5% of patients with sacral duct cysts will present with protrusion of the cyst into the pelvic cavity. These patients are often accompanied by bone destruction of the sacrum and enlargement of the anterior sacral foramen. Clinical manifestations are mainly pain in the lumbar, pelvic, perineal, and sacrococcygeal regions, or manifest as sciatica, with some patients having combined bladder, bowel, or sexual dysfunction. Many patients tend to go to gynecology or anorectology first, and it can easily lead to misdiagnosis. In these patients, it is recommended to improve the MRI scan of the sacrococcygeal spine in multiple planes, especially the reconstruction of MRI in coronal position, which is quite helpful to determine the origin of the cyst, the number of cysts and the location of the nerve root outlet. In principle, patients with huge sacral cysts protruding into the pelvic cavity and with corresponding clinical symptoms should be actively treated by surgery as early as possible. For patients with no obvious symptoms, surgery should also be recommended. Cysts protruding into the pelvic cavity tend to progress faster due to bone defects, and the larger the cyst is, the more difficult it is to treat clinically. 3. Such patients often first consult gynecology or anorectology because of their symptoms, and if they are misdiagnosed and undergo open surgery as simple pelvic cysts or adnexal cysts, the consequences are unthinkable. These cysts originate from sacral nerve roots and the cyst leakage is located in sacral nerve sheath in sacral canal, so it is most important to seal the leakage of cysts at this time in order to finally achieve the purpose of radical treatment. Frequently asked questions: A. Do I need surgery if I have a huge sacral cyst but no symptoms at present? A: Yes. Giant sacral cysts, which have destroyed bone, can be operated upon detection to stop the onset of symptoms. Second, how to differentiate adnexal cysts or pelvic cysts from sacral canal cysts. A: It is recommended to consult the lumbosacral nerve center of neurosurgery. Such patients are recommended to perfect a multiplanar MRI scan of the sacrococcygeal spine. Especially, the reconstruction of MRI in coronal position can be quite helpful in determining the origin of cyst, the number of cysts and the location of nerve root exit. Third, can minimally invasive laparoscopic surgery be performed for sacral cysts that protrude into the abdominal cavity? A: No. Such cysts protruding into the abdominal and pelvic cavities have their roots from the nerve sheaths in the sacral canal, and surgery simply for the cysts is often ineffective and recurs early. It is more likely to cause intraoperative nerve root injury and persistent postoperative cerebrospinal fluid leakage with unimaginable consequences. Therefore, we recommend to perform sacral canal cyst blocking surgery in order to solve the problem at its root.