The objective of this study was to ascertain the annual incidence density of thyrotoxicosis in children under the age of 15 years in Denmark in 1982-1988. The design was based on computerized hospital registration of patient admittances in all departments of paediatrics and internal medicine of Denmark (Faroe Islands and Greenland excluded). Fifty-six children (48 girls and 8 boys) had a confirmed diagnosis of thyrotoxicosis, giving a national incidence density of 0.79/100,000 person-years. In children aged 0-4 years the incidence was very low (0.1/100,000), with no sex difference. In boys aged 5-9 years a similar low incidence was found, while in boys aged 10-14 years the incidence increased to 0.48/100,000. In girls aged 5-9 years the incidence increased to 0.96/100,000, reaching a maximum of 3.01 in the 10-14-year-old girls. In children of > 4 years of age a female preponderance of 6.7:1 was significant. It is concluded that thyrotoxicosis is a rare disorder in Danish children under the age of 15 years, and the incidence increases with age. Female preponderance is significant from early childhood.