An in vivo sensor system for direct measurement of pressure in the human urinary bladder is developed. The core component in the system is a small-sized and highly sensitive piezoresistive MEMS pressure sensor element integrated in a sensor catheter. The sensor catheter is wired to an external module for biasing, sampling, conversion and storage of sensor measurements. Our solution provides a target sensor placed directly into the urinary bladder and a reference sensor placed outside the bladder wall through a suprapubic and minimally invasive technique. Physiological recordings through natural filling and emptying cycles of the bladder are achievable. The case report from the first 17-hours investigation in a patient is presented in this paper. It reveals that the procedure was successful and no complications occurred. The patient expressed good experience during the participation. A functionality test shows that the percutaneous pressure sensor system responds immediately to external pressure stimuli.