A method of detecting tissue oedema in a subject. The method includes determining a measured impedance for first and second body segments. An index indicative of a ratio of the extra-cellular to intra-cellular fluid is then calculated for each body segment, with these being used to determine an index ratio based on the index for the first and second body segments. The index ratio can in turn be used to determine the presence, absence or degree of tissue oedema, for example by comparing the index ratio to a reference or previously determined index ratios.