A fully implantable telemetric differential pressure sensor has been developed which is incorporated with a shunt valve system. It is a unique single diaphragm device that allows in vivo confirmation of zero-point and pressure calibrations. The telemetry is implemented by a resonant circuit in the sensor and a radio frequency detector outside the body, and the in vivo pressure calibration is derived from a pressure-balancing method. The pressure sensor has been implanted with shunts in several patients, and is shown to be valuable both for measurements of intraventricular pressure and for determining proper function of hydrocephalus shunt valves and catheters.