A 27-year-old woman with severe chronic hemolytic anemia was found to have reduced red cell hexokinase activity when the degree of reticulocytosis was considered. This enzyme had normal pH-dependent activity, normal Km for glucose, fructose, and mannose, normal Km for Mg adenosine triphosphate (ATP)2- and Ki for glucose-1,6-diphosphate. Furthermore, the pH-dependence and orthophosphate dependence of Ki for glucose-1,6-diphosphate were normal. However, this hexokinase was inactivated rapidly at 44 degrees C. No abnormalities were found in the red cell hexokinase isozymic pattern when it was compared with the profile obtained from cells of similar age. The hexokinase specific activity was reduced in all the red blood cell fractions obtained by density gradient ultracentrifugation; a marked difference in the distribution of cells through the gradient was evident. Among the glycolytic intermediates, a significant decrease of 2,3-diphosphoglycerate was evident. ATP and glucose 6-phosphate were also reduced when compared with cells of similar. Glucose consumption of the hexokinase-deficient cells decreased, but the rate of glucose metabolized through the hexose monophosphate shunt was unchanged. Although the total hexokinase activity in lymphocytes was only reduced by 37%, a marked hexokinase deficiency was detected in blood platelets (20% to 25% of normal activity). The parents and one of two siblings of the patient were heterozygous for the defect, with 66% to 74% of normal erythrocyte hexokinase activity and reduced heat stability of the enzyme. These results, when compared with those obtained in previously reported cases of hexokinase deficiency, provide further evidence of the broad phenotypic variability that characterizes this disorder. Furthermore, it is suggested that failure of energy generation is probably the primary cause of hemolytic anemia in hexokinase deficiency.