A silicone catheter manufactured originally for ventricular fluid drainage was used for continuous monitoring of intracranial pressure. It can be placed under the dura mater immediately before replacing the craniotomy flap or by introduction through an ordinary burr hole. Pressure is monitored isovolumetrically by connecting the catheter to a pressure transducer mounted at the bedside. This technique has been used in 34 neurosurgical patients for periods of up to 12 days without any untoward effects. This monitoring system by a subdurally placed silicone catheter is simple, safe, inexpensive, and acceptably reliable. It has a place in clinical practice, particularly when the lateral ventricle cannot be tapped because of its deviation or collapse.