An investigation of the physical state of the human thorax and the associated thoracic electrical impedance is presented. From an experimental study with 60 subjects (23 without pulmonary disease and 37 with obstruction or restriction) it is concluded that the percentage change in the measured input impedance from the predicted value is a good index to estimate the size of oedema and the physical state of the lungs. These results are in good agreement with the theoretical analysis of the problem and the patient's physical state.