Abstract: Osterporosis apparatus for measuring ultrasonic characteristic(s) of a patient's bone includes two ultrasonic transducers spacedly arranged in a respective head in the apparatus for ultrasonic transmission from one to the other; circuitry for controlling transmission from the one transducer, measuring the reception at the other and providing an output indicative of the ultrasonic characteristic(s), the apparatus including a fluid system having two diaphragms arranged in the respective heads so that there is a fluid path from each transducer to its diaphragm and a gap between the diaphragms which is occupied in use by the patient's bone.