Aim: The aim of this study was to determine in a Danish nationwide study whether the incidence rate (IR) of thyrotoxicosis in children below 15 years of age has increased between 1998 and 2012 and to compare the results with previously published national data from 1982 to 1988. Furthermore, we intended to conduct a descriptive study of children diagnosed with Graves' disease (GD) between 2008 and 2012. Methods: Children diagnosed with thyrotoxicosis between 1998 and 2012 were identified through the Danish National Patient Registry. All medical records were reviewed to verify the diagnosis. Additional data were collected on children diagnosed with GD in 2008-2012. Results: In total, 237 patients with juvenile thyrotoxicosis (JT) were identified. The overall IR in 1998-2012 was 1.58/100,000 person-years and has increased significantly from 0.79/100,000 person-years in 1982-1988 (p < 0.001). The IR has continued to increase during the recent 15 years (1.31-1.83/100,000 person-years), with a 12.5% increase with each 5-year time period. The IR increased with age and female sex (p < 0.001). The descriptive study included 79 children with GD, presenting with a wide spectrum of clinical features. Remission occurred in 13.9%. Conclusion: The IR of JT has increased significantly since 1982-1988 and has continued to increase during the recent 15 years. The overall IR was 1.58/100,000 person-years during 1998-2012.