Thiamazole is an antithyroid drug, largely used in the treatment of Graves's disease. One side effect is granulocytopenia up to agranulocytosis, that makes necessary a periodic blood count and withdrawal of medication once it was discovered. The pathological profile of those cases with hematological disturbances is still unclear. It seems that autoimmunity, high doses of thiamazole increases the risk. Age is not a risk factor. If the chance is higher in those patients who are at risk of hematological malignancy is still debated.