Context: Hyperthyroidism affects all age groups, but epidemiological data on children are scarce. Objective: To conduct a nationwide epidemiological survey of hyperthyroidism in children and adolescents. Design: A cross-sectional descriptive study. Setting: Identification of records corresponding to reimbursements for antithyroid drugs in the French national health insurance database. Participants: All cases of childhood hyperthyroidism (ages 6 months to 17 years) in 2015. Main Outcome Measures: national incidence rate estimated using a nonlinear Poisson model and spatial distribution of cases. Results: a total of 670 cases of childhood hyperthyroidism were identified. Twenty patients (3%) had associated autoimmune or genetic diseases, with type 1 diabetes and Down syndrome being the most common. The annual incidence for 2015 was 4.58 per 100,000 person-years (95% CI 3.00 to 6.99 per 100,000). Incidence increased with age in both sexes. This increase accelerated after age 8 in girls and age 10 in boys and was more pronounced in girls. About 10% of patients were affected before the age of 5 years (sex ratio 1.43). There was an interaction between age and sex, with the effect of being female increasing with age: girls were 3.2 times more likely to be affected than boys in the 10–14-year-old age group and 5.7 times more likely to be affected in the 15–17-year-old age group. No conclusions regarding spatial patterns emerged. Conclusion: these findings shed light on the incidence of hyperthyroidism and the impact of sex on this incidence during childhood and adolescence. The observed incidence was higher than expected based on the results published for earlier studies in Northern European countries.