From the national patient registry, we identified all patients with non-valvular atrial fibrillation or atrial flutter in the period 1997-2006. We defined non-valvular atrial fibrillation by a discharge diagnosis of atrial fibrillation or atrial flutter (diagnosis code I48), no previous diagnoses of mitral or aortic valve disease (394-396, 4240, 4241, I05, I06, I34, I35), and no mitral or aortic valve surgery (surgical procedure codes KFK, KFM, KFP), as done previously.17 (link) Because drug treatment may be changed or intensified in relation to hospital admission, we started follow-up seven days after discharge. We excluded patients if they died or had a thromboembolism in this seven day quarantine period. We identified drug treatment status from prescription claims from 180 days before discharge to seven days after discharge, and we excluded patients if they had received vitamin K antagonists (medicine code B01AA) or heparins (B01AB) (fig 1). We censored patients at time of death or at the end of the follow-up periods—that is, at one, five, and 10 years.