A new variant of congenital hemolytic anemia associated with stomatocytosis, reticulocytosis, decreased osmotic fragility, type I autohemolysis and shortened erythrocyte survival without specific splenic sequestration was discovered in three siblings of Swiss-German ancestry. Increased intracellular sodium (two to three times normal) and slightly decreased intracellular potassium were detected. Total sodium efflux was eight-fold greater than normal but total potassium influx was normal and ouabain-sensitive potassium influx was decreased. The ouabain-sensitive sodium efflux: potassium influx ratio was 26:1 rather than the 3:2 ratio noted in normal cells. The consanguineous parents, four other siblings, and 44 other family members had mild stomatocytosis, reticulocytosis, and, when studied, decreased osmotic fragility, increased autohemolysis, intermediate abnormalities of cation content, cation flux, and moderate shortening of erythrocyte survival. Autosomal dominant inheritance was suggested. No abnormalities of RBC enzymes, hemoglobin or lipids were observed. No abnormalities of membrane protein were detected on acrylamide gel. Substrate depletion of these hypermetabolic cells resulted in intracellular dehydration with potassium loss in excess of sodium gain and decreased deformability. Although the exact nature of the defect responsible for hemolysis is unknown, this syndrome differs from other hereditary hemolytic anemias associated with stomatocytosis.