Recent studies indicate that the hemolytic process in hereditary spherocytosis (HS) results from an intrinsic defect in the red cell membrane. The HS red cell is abnormally permeable to sodium ions, accumulating sodium and water intracellularly during incubation in vitro or after entrapment in splenic sinusoids. This is partially responsible for their characteristic excessive osmotic fragility and their propensity to premature destruction in the spleen. To compensate, active pumping of sodium is increased, which in turn stimulates membrane phospholipid turnover. In HS red cells, as in normal cells made hyperpermeable by various means, increased turnover of membrane phospholipids appears to render the cell membrane vulnerable to lipid depletion. The diminution in surface material which results in HS red cells produces microspherocytosis, the morphologic hallmark of the disease.