Hereditary spherocytosis (HS) is a genetically determined abnormality of red blood cells. It was first described in 1871 and is the most common cause of inherited haemolysis in Europe and North America within the Caucasian population, with an incidence of 1 in 5000 births.1 It seems to be less common in African-American and south-east Asian people. However, studies of osmotic fragility in blood donors suggest the existence of an extremely mild or subclinical form raising the prevalence of HS to 1 in 2000.2–4 The main molecular defects in hereditary spherocytosis are heterogenous. A usual feature of the red blood cell is disruption in vertical interactions between the membrane skeleton and the lipid bilayer components (Figure 1). When these interactions are compromised, loss of cohesion between bilayer membrane skeleton occurs, leading to the formation of spherocytes with reduced deformability that are selectively retained and damaged in the spleen.5 The clinical severity of HS varies from symptom-free carrier to severe haemolysis. Mild HS can be difficult to identify because individuals may have normal haemoglobin and bilirubin. Occasionally, mild HS can be exacerbated by illnesses that cause splenomegaly, such as infectious mononucleosis, as well as pregnancy or exercise.6,7 Partial deficiencies of the specific membrane proteins, reductions in membrane mechanical stability and loss of membrane surface area are directly related and are major features in determining the heterogeneous clinical manifestations of hereditary spherocytosis4,8,9 The imposition of cardiopulmonary bypass (CPB)