A new unstable haemoglobin, Hb Madrid (β115 [G17] Ala →·Pro), has been found in a Spanish boy suffering from a moderately severe haemolytic anaemia. This haemoglobin is the eighth example of haemoglobin instability resulting from insertion of a proline residue into a helical segment of the globin chain. The clinical and haematological features of the affected carrier are described, and the significance of substituting the normal amino acids by proline is discussed in terms of the stability of haemoglobin.