Aortic valve replacement in severe aortic stenosis (AS) with a low left ventricular ejection fraction (EF) is associated with high perioperative mortality. The aim of this study was to assess the prognostic value of preoperative atrial fibrillation (AF) in patients with AS and low EFs who undergo aortic valve replacement. Eighty-three consecutive patients with severe AS (area <1 cm2) and low EFs (< or =35%) were prospectively included. Perioperative mortality was 12%. Twenty-nine patients (35%) had preexisting paroxysmal or permanent AF. Perioperative mortality was higher in the AF group than in the non-AF group (24% vs 5.5%, p = 0.03). Preoperative AF was identified as an independent predictor of perioperative mortality (odds ratio 7.5, 95% confidence interval 1.19 to 47.06, p = 0.03). Five-year overall survival was lower in the AF group than in the non-AF group (47% vs 77%, p = 0.0017). Associated multivessel coronary artery disease and preoperative AF were identified as 2 independent predictors of overall mortality. In conclusion, in patients with AS with low left ventricular EFs, preoperative AF is associated with higher operative risk and lower postoperative survival. The presence of AF in patients with severe AS and low EFs should be taken into account for operative risk stratification, along with low pressure gradient and associated multivessel coronary artery disease.