Total and differential leucocyte countings, mobilization of the marrow granulocyte reserve with hydrocortisone and growth of blood colony-forming cells (CFU-C) in agar culture have been evaluated in 17 patients with Graves’ disease before therapy. Moreover, the myeloid differentiation of blood CFU-C in liquid culture has been studied in 4 patients. Neutropenia was found in only 1 patient (5.9%), while the marrow granulocyte reserve was reduced in 7 out of 15 patients (46.7%). Blood concentration of CFU-C and myeloid differentiation in liquid culture were normal in all patients studied. These data show an abnormal granulopoiesis in a high percentage of patients with untreated Graves’ disease. The mechanisms accounting for the reduction of the marrow granulocyte reserve in patients with Graves’ disease are discussed.