Negative-pressure hydrocephalus is a rare condition with the development of symptomatic hydrocephalus despite subnormal intracranial pressure (ICP). The etiology remains unclear. Some authors proposed that the differential pressure between the ventricular space and the subarachnoid space over cerebral convexity leads to the development of ventriculomegaly, namely as the transmantle pressure theory. A 49-year-old patient with a left Sylvian fissure arachnoid cyst underwent several surgeries including cystoperitoneal shunts and fenestrations of the cyst. The patient developed a cerebrospinal fluid fistula from the cranial wound was complicated by bacterial meningitis. Consequently, the shunt was removed, and external cyst drainage was placed. After 9 days, the patient developed acute hydrocephalus requiring external ventricular drainage (EVD). Two days later, after overdrainage of the external cyst drain, the patient suffered neurologic deterioration. The ICP measured by the EVD was –4 cm H 2 O, and a computed tomography scan demonstrated progression of the hydrocephalus. The external cyst drainage was shut off and the EVD level was adjusted to produce between 5 and 10 mL/hour of cerebrospinal fluid under a subatmospheric pressure set at –5 cm H 2 O, and gradually raised in increments of 1 cm every 3 days until a positive ICP occurred. Once clinical and radiographic stability was accomplished, a programmable ventriculoperitoneal shunt was inserted set to 30 mm H 2 O. A marked clinical and radiologic improvement was observed in the follow-up. This negative-pressure hydrocephalus case report supports the main role of the transmantle pressure theory. The subatmospheric EVD method and a low-pressure valve resulted in excellent clinical and radiographic outcomes. [ABSTRACT FROM AUTHOR]