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To investigate the association between estimated cardiorespiratory fitness (eCRF) and risk of atrial fibrillation (AF), and examine how long-term changes in eCRF affects the AF risk.This prospective cohort study includes data of 39,844 men and women from the HUNT2 (August 15, 1995 to June 18, 1997) and the HUNT3 study (October 3, 2006 to June 25, 2008). The follow-up period was from HUNT3 until AF diagnosis or November 30, 2015. The AF diagnoses were retrieved from hospital registers and validated by medical doctors. A nonexercise test based on age, waist circumference, resting heart rate and self-reported physical activity was used to estimate CRF. Cox regression was performed to assess the association between eCRF and AF.The mean age was 50.6 ± 14.6 yr for men and 50.2 ± 15.2 yr for women. Mean follow-up time was 8.1 yr. One thousand fifty-seven cases of AF were documented. For men, the highest risk reduction of AF was 31% in the fourth quintile of eCRF when compared with the first quintile (hazard ratio [HR], 0.69; 95% confidence interval [CI], 0.53–0.89). For women, the highest risk reduction was 47% in the fifth quintile when compared with the first quintile (HR, 0.53; 95% CI, 0.38–0.74). One metabolic equivalent increase in eCRF over a 10-yr period was associated with 7% lower risk of AF (HR, 0.93; 95% CI, 0.86–1.00). Participants with improved eCRF had 44% lower AF risk compared with those with decreased eCRF (HR, 0.56; 95% CI, 0.36–0.87).The eCRF was inversely associated with AF, and participants with improved eCRF over a 10-yr period had less risk of AF. These findings support the hypothesis that fitness may prevent AF.