Children with obesity have a cardio-metabolic risk profile that may predispose them to cardiovascular diseases. We examined the associations between childhood body mass index (BMI) levels and changes on the risk of atrial fibrillation and flutter (AFF) in adulthood.We conducted a population-based cohort study of Danish schoolchildren aged 7-13 years born from 1930-1989. Among 314,140 children, 17,594 were diagnosed with AFF as adults. In both men and women, above-average BMIs in childhood were associated with increased risks of AFF. Children who were persistently heavy at ages 7 and 13 years, and children whose BMI increased from the internal 25th-75th percentiles or from the internal 75.1 th-90th percentiles between ages 7 and 13 years had higher risks of AFF in adulthood than children whose BMIs remained in the internal 25th-75th percentiles at both ages. A decrease in BMI percentile categories between 7 and 13 years of age reduced risks of AFF in adulthood, with risks of AFF reverting to levels similar to those in the reference group for women, but not for men.In conclusion, risks of AFF in adulthood increased with higher childhood BMIs. Remission from overweight by age 13 years reduced AFF risks, especially in women.