Two children with DIDMOAD syndrome (diabetes insipidus, diabetes mellitus, optic atrophy, deafness) developed a megaloblastic and sideroblastic anemia, neutropenia, and borderline thrombocytopenia. Plasma thiamine concentration was low in one patient and normal in the other; in both children, thiamine pyrophosphate in erythrocytes and thiamine pyrophosphokinase activity were lower than the lowest values observed in control subjects. A month after institution of treatment with thiamine, the hematologic findings had returned to normal and the insulin requirements had decreased. Withdrawal of thiamine repeatedly induced relapse of the anemia and an increase in insulin requirements. We propose that an inherited abnormality of thiamine metabolism is responsible for the multisystem degenerative disorder known as DIDMOAD syndrome.