We analyzed the possible association between aldosterone synthase (CYP11B2) T-344C polymorphism, which is associated with increased aldosterone activity, and the prevalence of atrial fibrillation (AF) in 196 consecutive patients who had symptomatic systolic heart failure (HF; left ventricular ejection fraction <40%) for > or =3 months before recruitment. Genomic DNA was extracted from peripheral blood leukocytes using a standard protocol. Subjects were genotyped for the CYP11B2 polymorphism using the polymerase chain reaction/restriction fragment length polymorphism approach. AF was present in 63 patients (33%) with HF. We found the -344 CC genotype to be a strong independent marker for AF. Almost 1/2 (45%) of patients with this genotype had AF compared with 1/4 (27%) with -344 TT and TC genotypes (p = 0.01). A multivariate stepwise logistic regression model that included age, gender, New York Heart Association class, CYP11B2 -344CC genotype, and echocardiographic measurements of left ventricular ejection fraction, left atrial dimension, left ventricular end-diastolic diameter, and mitral regurgitation severity showed that the CYP11B2 CC genotype (adjusted for age and left atrial size) was an independent predictor of AF (adjusted odds ratio 2.35, 95% confidence interval 1.57 to 3.51, p = 0.03). In conclusion, CYP11B2 T-344C promoter polymorphism predisposes to clinical AF in patients with HF.