BACKGROUND:This register-based observational study compares dabigatran to warfarin for secondary stroke prevention in atrial fibrillation patients among both "new starters" on dabigatran and "switchers" to dabigatran from warfarin. METHODS:We identified, in nationwide Danish registries, 2398 patients with atrial fibrillation and a history of stroke/transient ischemic attack, making a first-time purchase of dabigatran 110 mg twice a day (bid; D110) and 150 mg bid (D150). Patients were categorized as either vitamin K antagonist (VKA) naive or experienced. Warfarin controls were identified using a complete (for VKA-naive dabigatran patients) or matched sampling approach (for VKA-experienced dabigatran patients). Subjects were followed for an average of 12.6 months for stroke and transient ischemic attacks. Confounder-adjusted Cox regression models were used to compare event rates between treatments. RESULTS:Among patients with a history of stroke/transient ischemic attack and prior VKA experience, switching to dabigatran was associated with an increased stroke/transient ischemic attack rate for both dabigatran doses compared with continuing on warfarin (D110 hazard ratio [HR] 1.99; 95% confidence interval [CI], 1.42-2.78; D150 HR 2.34; 95% CI, 1.60-3.41). Among prior stroke/transient ischemic attack patients who were new starters on dabigatran or warfarin, the rate of stroke/transient ischemic attack for both doses of dabigatran was similar to or lower than warfarin (D110 HR 0.64; 95% CI, 0.50-0.80; D150 HR 0.92l; 95% CI, 0.73-1.15). CONCLUSIONS:In this register-based study, VKA-experienced patients with a history of stroke or transient ischemic attack who switched to dabigatran therapy had an increased rate of stroke compared with patients persisting with warfarin therapy.