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Aim: To examine trends in all body mass index (BMI) groups in children from 1936 to 2011. Methods: We included 197 694 girls and 201 276 boys from the Copenhagen School Health Records Register, born between 1930 and 1996, with longitudinal weight and height measurements (6–14 years). Using International Obesity Task Force criteria, BMI was classified as underweight, normal‐weight, overweight and obesity. Sex‐ and age‐specific prevalences were calculated. Results: From the 1930s, the prevalence of underweight was stable until a small increase occurred from 1950 to 1970s, and thereafter it declined into the early 2000s. Using 7‐year‐olds as an example, underweight changed from 10% to 7% in girls and from 9% to 6% in boys during the study period. The prevalence of overweight plateaued from 1950 to 1970s and then steeply increased from 1970s onwards and in 1990–2000s 15% girls and 11% boys at 7 years had overweight. The prevalence of obesity particularly increased from 1980s onwards and in 1990–2000s 5% girls and 4% boys at 7 years had obesity. These trends slightly differed by age. Conclusion: Among Danish schoolchildren, the prevalence of underweight was greater than overweight until the 1980s and greater than obesity throughout the period. Thus, monitoring the prevalence of childhood underweight remains an important public health issue. [ABSTRACT FROM AUTHOR]