BACKGROUND Agranulocytosis is a serious side effect of antithyroid drugs. OBJECTIVE To ascertain the knowledge of patients and review the quality of information available on the internet. METHODS A questionnaire survey was performed for patients receiving antithyroid drugs. Patients attending endocrine clinics who were receiving antithyroid drug treatment (group A, n = 33) were interviewed. A further national cohort of patients (group B, n = 100) treated with antithyroid drugs, participated in an online survey. RESULTS 60.9% of responders were not aware of the common symptoms of agranulocytosis. 18.6% had never received any information about side effects. Of the 108 patients who recalled receiving information, 30% rated the quality as 'poor' or 'not good at all'. Structured interviews of group A patients revealed that almost half (45.5%, 15/33) had experienced symptoms that could be indicative of agranulocytosis, but only 53.3% (8/15) had a blood count checked. A review of 20 selected patient information internet sites revealed a significant variation in advice given to patients. CONCLUSIONS Inadequate knowledge about agranulocytosis among patients receiving antithyroid drug treatment is common. The available information on the internet is variable and inconsistent.