Perineural cysts have become a common incidental finding during lumbosacral magnetic resonance (MR) imaging. Only some of the symptomatic cysts warrant treatment. The authors describe the successful operative treatment of a patient with, to the best of their knowledge, the largest perineural cyst reported to date. A 29-year-old woman had been suffering from long-standing constipation and low-back pain. During an obstetric investigation for infertility, the clinician discovered a huge presacral cystic mass. Computed tomography myelography showed the lesion to be a huge Tarlov cyst arising from the left S-3 nerve root and compressing the ipsilateral S-2 nerve. The cyst was successfully treated by ligation of the cyst neck together with sectioning of the S-3 nerve root. Postoperative improvement in her symptoms and MR imaging findings were noted. Identification of the nerve root involved by the cyst wall, operative indication, operative procedure, and treatment of multiple cysts are important preoperative considerations.