Cation permeability and lipid composition have been studied in the red cells of five patients with various features of the hereditary stomatocytosis syndrome. Hemolysis was compensated in four patients, and only one patient was anemic. Cell NA+ was increased an average of 3 mueq per ml cells and cell K+ decreased 14 mueq per ml cells. Both active and passive fluxes of Na+ and K+ were increased by two to six times normal. Tritiated ouabain binding was increased an average of 2.5- fold, suggesting a proportionally greater number of cation pumps per cell. The coupling ratio of active Na+:K+ fluxes was normal (3:2). Calcium permeability was increased compatible with the degree of reticulocytosis, and cell Ca2+ content was normal. The lowered sum of Na+ plus K+ was associated with a high MCHC and low cell water. When examined in wet preparations, red cells assumed either a bowl-shaped or an irregular contour, and they appeared as target cells on dry smears. Only when cell water was increased in hypotonic media were stomatocytes seen on smear. The total lipid content of red cells was increased in four patients, although it was normal in one. The mole ratio of cholesterol to phospholipid was always normal; however, phospholipid analysis showed an increased proportion of phosphatidyl choline. The abnormal cells were osmotically resistant due to both an increased membrane surface area and a low total cation content. These patients show two hallmarks of hereditary stomatocytosis: bowlshaped red cells observed on wet preparations and a marked increase in Na+ and K+ permeability. The heterogeneity of this syndrome in our patients and in others reported with hereditary stomatocytosis appears to result from (1) variability in the increase in surface area which results from an excess of membrane lipid content, particularly phosphatidylcholine, and (2) a variability in cell water content which may be either decreased or increased as a result of changes in the sum of Na+ plus K+ ions.