A non-invasive method and apparatus for monitoring the bladder volume in humans or animals. The monitoring measurements are based on the analysis of ultrasound signals from at least two individual ultrasound transducer arrays. Each one individual ultrasound transducer array has a contact face, which is arranged in order to transmit an ultrasonic signal through an abdominal surface of an individual being monitored. The method and apparatus includes the arranging of each individual transducer array on the abdominal surface around an axis which extends from a point on the abdominal surface through a point on or within the urinary bladder. The method and apparatus further comprises that each individual transducer array is arranged to scan the bladder using a two-dimensional scan plane which extends radially from the transducer array through the bladder.