BACKGROUND : Giant Tarlov cysts are always symptomatic and mimic many serious pathologic entities. We present the largest, to our knowledge, reported Tarlov cyst. METHODS : A 33-year-old woman with Marfan syndrome suffered from right kidney hydronephrosis because of ureter obstruction, for which she was treated with nephrostomy. Her neurologic examination was unremarkable. The role of magnetic resonance imaging in the management of this case is described. Absence of intractranial hypotension symptoms after cerebrospinal fluid (CSF) overdrainage suggested the presence of a valve-like mechanism. The patient was planned for surgical cyst remodeling by the application of titanium clips. The cyst's neck was exposed through an L5-S2 laminectomy. L5 and S1 laminae were severely eroded. CSF leaked out of the underlying, bulging, and thinned dura at each attempt for clip application. Intraoperatively, multiple responses from the S1, S2, and S3 roots were simultaneously recorded at each stimulation. Therefore, we decided to abandon this technique and performed a nonwatertight duroplasty followed by a layered wound closure instead. A week later, the patient received a lumbar-peritoneal shunt. The patient remained neurologically intact, the cyst shrunk, and the nephrostomy was removed. CONCLUSIONS : Indirect evidence was helpful to assess for the presence of a valve-like mechanism. Intraoperatively, the surgeon must keep on high alert for sacral erosion to avoid inadvertent dural tear and rootlet injury. Finally, lumboperitoneal diversion remains a valid alternative in the management of our giant Tarlov cyst because it reduced the intracystic pressure that resulted in cyst regression.