A method of non-invasively monitoring the respiration of a patient comprises: transmitting ultrasound into the body toward an internal structure of the patient's body, the internal structure being one of the liver, the spleen or a kidney; selecting a depth range; measuring the phase of ultrasound echo signals from the internal structure at multiple points along the depth range for at least a first and a second echo signal, the first and second echo signals being received at different times; detecting the motion of the internal structure within the patient's abdomen by reference to differences in the measured phase between the first and the second echo signals; and thereby monitoring the respiration of the patient by associating movement of the internal structure with movement caused by respiration.