This study was aimed at establishing the importance of routine monitoring of white blood cell counts in patients with Graves' disease receiving antithyroid drug treatment. In the 12-year period from 1975 to 1987, 15,398 patients with Graves' disease receiving treatment with antithyroid drugs were seen at our clinic. Of these, 55 (0.4%) were found to have agranulocytosis. Agranulocytosis was defined as a granulocyte count of 0.5 x 10(9)/L or less. In only 12 of the 55 patients was agranulocytosis detected after the occurrence of infection (symptomatic; classic agranulocytosis). The remaining 43 patients were asymptomatic when agranulocytosis was detected during routine white blood cell count monitoring. However, 14 of these 43 patients became symptomatic several days after withdrawal of antithyroid drug treatment despite antimicrobial treatment (asymptomatic to symptomatic). Twenty-nine patients who were treated appropriately had no symptom of infection throughout the course of the disease, despite the absence of or an extremely small number of granulocytes in circulation (asymptomatic). These results suggest that a "routine monitoring" of the white blood cell count could be the most effective way of predicting and detecting agranulocytosis due to antithyroid drug treatment.