BACKGROUND: Strokes develop even in patients with low CHADS(2) scores, and the left atrial appendage (LAA) is the embolic source 90% of the time. We focused on the LAA morphology as a new predictor of strokes. OBJECTIVE: We aimed to clarify the anatomical characteristics of the LAA for risk stratification of strokes in non-valvular atrial fibrillation (AF) patients with low CHADS(2) scores. METHODS: Among 80 patients who underwent catheter ablation of AF with contrast-enhanced computed tomography (CT), the LAA characteristics were compared between 30 patients with prior histories of strokes and 50 age-matched controls. The LAA anatomy was classified into 4 types: "cactus", "cauliflower", "chicken wing", and "windsock", discriminated by the CT measurements of the length, angle, and number of lobes of the LAA. RESULTS: The average CHADS(2) score did not significantly differ between the stroke and control patients (0.8±0.8 vs. 0.6±0.7, P=0.277). Eight stroke patients (26.7%) had CHA(2)DS(2)-VASc scores of 0. The left atrial size, LAA flow velocity, left ventricular function, and serum brain natriuretic peptide level were also unable to predict strokes. However, a "cauliflower" LAA, defined as a main lobe of less than 4cm long without forked lobes, was significantly more common in stroke patients (odds ratio (OR): 3.857; 95% confidence interval [CI]: 1.482-10.037; P=0.005). The CHA(2)DS(2)-VASc score-adjusted logistic regression analysis revealed the "cauliflower" LAA as an independent predictor of a stroke (OR: 3.355; 95% CI: 1.243-9.055; P=0.017). CONCLUSIONS: The LAA anatomy might be useful to predict strokes in non-valvular AF patients with low CHADS(2) scores.