Background: We found an increase in the incidence rate (IR) of childhood thyrotoxicosis (CT) during the 1990s in central Sweden. The optimal treatment method for CT is a subject that is still debated upon. Objectives: To investigate whether the increase in IR of CT in Sweden persists and to study the treatment outcome. Method: Children <16 years of age diagnosed with CT during 2000–2009 and living in 1 of 5 counties in central Sweden were identified retrospectively using hospital registers. Data on clinical and biochemical characteristics and outcomes of treatment were collected from medical records. The corresponding data from 1990 to 1999 were pooled with the new data. Results: In total, 113 children were diagnosed with CT during 1990–2009 in the study area. The overall IR was 2.2/100,000 person-years (95% CI 1.2–2.5/100,000 person-years). The IR was significantly higher during 2000–2009 than during 1990–1999 (2.8/100,000 [2.2–3.6] vs. 1.6/100,000 person-years [1.2–2.2], p = 0.006). The increase was significant for both sexes. Seventy percent of the patients who completed the planned initial treatment with antithyroid drugs (ATDs) and were not lost to follow-up relapsed within 3 years. Boys tended to relapse earlier than girls (6.0 months after drug withdrawal [95% CI 1.9–10.0] vs. 12.0 months [95% CI 6.8–17.3], p = 0.074). Conclusions: The IR of CT is increasing in both girls and boys. Relapse rate after withdrawal of ATD treatment is 70%. Boys tend to relapse earlier than girls, and this needs to be further investigated.