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Objectives: Mild cognitive impairment (MCI) is regarded as a precursor to dementia, but not all patients with MCI actually develop dementia. As Alzheimer and vascular dementia are thought to share many common etiopathogenetic mechanisms, we investigated whether the vascular risk factor atrial fibrillation affects the risk of conversion to dementia for different MCI subtypes diagnosed according to international criteria. Methods: One hundred and eighty elderly outpatients with MCI and 431 elderly outpatients with a normal cognition were followed up for a mean of 3 and 4 years, respectively. The risk of conversion to dementia associated with atrial fibrillation was studied in both samples using a Cox proportional hazards model adjusted for socio-demographic and medical variables. Results: Overall conversion rate to dementia was 10.5 (8.0–13.8 per 100 person-years) in the MCI group and 2.2 (1.5–3.1 per 100 person-years) in the normal cognition group. Atrial fibrillation was significantly associated with conversion to dementia [hazard ratio (HR): 4.63; 95% confidence interval: 1.72–12.46] in the MCI group but not in the cognitively normal group (HR: 1.10; 95% confidence interval: 0.40–3.03). Discussion: Current diagnostic criteria for MCI subtypes define heterogeneous populations, but atrial fibrillation can be useful in identifying people with increased risk of conversion to dementia. [ABSTRACT FROM AUTHOR]