Introduction: The incidence of hyperthyroidism has been reported in various countries to be 23-93/100000 inhabitants per year. This extended study has evaluated the incidence for similar to 40% of the Swedish population of 9 million inhabitants. Sweden is considered to be iodine sufficient country. Methods:All patients including children, who were newly diagnosed with overt hyperthyroidism in the years 2003-2005, were prospectively registered in a multicenter study. The inclusion criteria are as follows:clinical symptoms and/or signs of hyperthyroidism with plasma TSH concentration below 0.2 mIE/l and increased plasma levels of free/total triiodothyronine and/or free/total thyroxine. Patients with relapse of hyperthyroidism or thyroiditis were not included. The diagnosis of Graves' disease (GD), toxic multinodular goiter (TMNG) and solitary toxic adenoma (STA), smoking, initial treatment, occurrence of thyroid-associated eye symptoms/signs, and demographic data were registered. Results:A total of 2916 patients were diagnosed with de novo hyperthyroidism showing the total incidence of 27.6/100 000 inhabitants per year. The incidence of GD was 21.0/100 000 and toxic nodular goiter (TNG=STA+TMNG) occurred in 692 patients, corresponding to an annual incidence of 6.5/100 000. The incidence was higher in women compared with men (4.2:1). Seventy-five percent of the patients were diagnosed with GD, in whom thyroid-associated eye symptoms/signs occurred during diagnosis in every fifth patient. Geographical differences were observed. Conclusion:The incidence of hyperthyroidism in Sweden is in a lower range compared with international reports. Seventy-five percent of patients with hyperthyroidism had GD and 20% of them had thyroid-associated eye symptoms/signs during diagnosis. The observed geographical differences require further studies.