Background: The incidence of acquired demyelinating syndromes (ADS) including multiple sclerosis (MS) has never been investigated in a Danish pediatric population. Objectives: We estimated the nationwide age- and sex-specific incidence of pediatric ADS including MS. Methods: Data were sourced from the Danish Multiple Sclerosis Registry, providing cases of pediatric MS for 1977–2015, and the National Patient Register, providing cases of ADS during 2008–2015. All medical records were reviewed to validate the register-based diagnoses. Results: We identified 364 cases of pediatric MS occurring during 1977–2015 (incidence rate = 0.79 per 100,000 person-years). MS was exceptionally rare before puberty, but the incidence rose considerably from 9 years in girls and 11 years in boys. The female-to-male ratio was 2.5; the median age at onset was 16 years (range = 7–17 years). The MS incidence rate was relatively stable through the study period. During 2008–2015, we identified 219 ADS cases. The incidence was 2.29 per 100,000 person-years with considerable differences in the age peaks for the separate ADS. Conclusion: The incidence rates of MS and other ADS in Denmark were higher than those reported for some other European countries. Referral bias and classification differences may account for this disparity, in particular the age-intervals and the definition of onset.