Severe acute hemoglobinemia and hemoglobinuria developed in a 17-year-old Japanese male with no history of exposure to any oxidizing agents. Examination of erythrocyte enzyme activity revealed that the patient's erythrocyte glutathione peroxidase (GSH-Px) activity was decreased to about a half that in control cells. Serum selenium (Se) concentration was within normal limits. Family studies showed that GSH-Px activity in the erythrocytes of two siblings was similarly decreased. It was suggested that the proband was suffering from a hereditary heterozygous GSH-Px deficiency, which appeared to be associated with acute hemolysis.