Long-term monitoring of intracranial pressure (ICP) was used as an aid for the management of four patients with pseudotumor cerebri. After the implantation of a small experimental pressure sensor in the skull, most of the subsequent (ICP measurements were made noninvasively by an external interrogator. During the initial study of the patient, baseline ICP recordings were made in the hospital before treatment. Pressure recordings on a 24-hour basis were continued during treatment, which, depending on the case, was with Diamox (acetazolamide), steroids, or the coperitoneal shunting. After discharge these patients returned weekly over 10 to 22 months for ICP measurement and for ophthalmological examination. Intracranial pressure before treatment showed irregular variations ranging from 100 to 500 mm H2O over a 24-hour period. The efficacy of treatments could be assessed in a few hours by the degree of ICP stabilization. Shunt malfunction was detected by a slow but continuous rise in pressure before full clinical signs were evident.