An 11-yr-old child with mild chronic hemolytic anemia was found to have decreased red cell hexokinase activity in spite of the reduced mean age of her red cell population. Similar decreases in red cell hexokinase activity were documented in the patient's parents and in one sib. The red cells were morphologically normal. Red cell 2,3-DPG levels were normal and ATP and glucose-6-phosphate levels were diminished. The kinetic properties, electrophoretic mobility, and thermal stability of the residual red cell hexokinase were normal or nearly so. Glucose consumption of the hexokinase-deficient cells was not appreciably decreased, probably because less of the potent inhibitor glucose-6-phosphate was present in the erythrocytes. It is likely, although not certain, that in this patient nonspherocytic hemolytic anemia resulted from hexokinase deficiency.