Abstract: A method and apparatus for measuring the intra-abdominal pressure within a patient. A reservoir is positioned between a drainage catheter (e.g. a urinary catheter or naso-gastric tube) and a collection container for the fluid being drained from the patient's body. The reservoir defines a constant volume and receives the withdrawn fluid through a manometer tube having a series of markings. The reservoir includes an air vent that ensures that the volume of fluid in the reservoir is held at atmospheric pressure when the reservoir is raised above the patient. When the reservoir is raised to a predetermined measurement height above the patient, the volume of fluid in the reservoir is returned to the patient. The height of the liquid column within the manometer tube provides a direct indication of the intra-abdominal pressure of the patient.