The risk for thrombosis is increased in patients with hereditary hydrocytosis, an uncommon variant of hereditary stomatocytosis. Erythrocytes from 2 patients with hydrocytosis were studied to gain insight into the mechanism of thrombosis in this disorder. Erythrocytes demonstrated abnormal osmotic scan ektacytometry and decreased erythrocyte filtration rates. There was also a mild increase in adherence of erythrocytes to endothelial monolayers in a micropipette assay. Adhesion of erythrocytes to the subendothelial matrix proteins thrombospondin and laminin, however, was not significantly increased. Percentages of hydrocytosis erythrocytes and reticulocytes with phosphatidylserine exposed on the outer surfaces were increased in both patients compared with healthy controls, indicating altered membrane phospholipid asymmetry. Increased phosphatidylserine exposure accelerating thrombin-forming processes has been proposed as a mechanism for thrombosis in sickle cell disease and β-thalassemia and may play a similar role in hereditary hydrocytosis.