The pleiotropic effects of statins have been suggested to prevent atrial fibrillation onset. We conducted a population-based case-control study using medical databases from Northern Denmark (population: 1.8 million) to examine the association between statin use and atrial fibrillation or flutter. We identified 51,374 patients with atrial fibrillation or flutter between 1999 and 2010 and 513,670 matched population controls. We collected data on statin prescriptions redeemed within 90 days (current users) or longer (former users) before the diagnosis date of atrial fibrillation or flutter. We stratified statin users by duration of exposure, determined by the number of days between first and last redeemed prescription before the diagnosis date (<365, 365-1094, and ≥1095 days). We used conditional logistic regression to compute odds ratios (ORs) and 95% confidence intervals (CIs), controlling for potential confounders. We defined people without previous statin use as never users (reference). A total of 7360 (14.3%) cases and 55,699 (10.8%) controls were current statin users. Among current users (adjusted OR: 0.96, 95% CI, 0.93-0.99), the preventive effect of statins on atrial fibrillation or flutter was related to duration of use: adjusted ORs decreased from 1.35 (95% CI, 1.28-1.42) for users who were prescribed statins for <365 days to 0.85 (95% CI, 0.81-0.89) for users who were prescribed statins for ≥1095 days compared with never users. For former users (adjusted OR: 0.94, 95% CI. 0.90-0.98), the ORs did not change with varying lengths of exposure. In conclusion, long-term statin use may reduce the risk of atrial fibrillation or flutter compared with never use.