Background—The influence of systolic and diastolic blood pressure (BP) on incident atrial fibrillation (AF) is not well studied among initially healthy, middle-aged women.

Methods and Results—A total of 34 221 women participating in the Women's Health Study were prospectively followed up for incident AF. The risk of AF across categories of systolic and diastolic BP was compared by use of Cox proportional-hazards models. During 12.4 years of follow-up, 644 incident AF events occurred. Using BP measurements at baseline, we discovered that the long-term risk of AF was significantly increased across categories of systolic and diastolic BP. Multivariable-adjusted hazard ratios for systolic BP categories (<120, 120 to 129, 130 to 139, 140 to 159, and 160 mm Hg) were 1.0, 1.00 (95% CI, 0.78 to 1.28), 1.28 (95% CI, 1.00 to 1.63), 1.56 (95% CI, 1.22 to 2.01), and 2.74 (95% CI, 1.77 to 4.22) (P for trend <0.0001). Adjusted hazard ratios across baseline diastolic BP categories (<65, 65 to 74, 75 to 84, 85 to 89, 90 to 94, and 95 mm Hg) were 1.0, 1.17 (95% CI, 0.81 to 1.69), 1.18 (95% CI, 0.84 to 1.65), 1.53 (95% CI, 1.05 to 2.23), 1.35 (95% CI, 0.82 to 2.22), and 2.15 (95% CI, 1.21 to 3.84) (P for trend=0.004). When BP changes over time were accounted for in updated models, multivariable-adjusted hazard ratios were 1.0, 1.14 (95% CI, 0.89 to 1.46), 1.37 (95% CI, 1.07 to 1.76), 1.71 (95% CI, 1.33 to 2.21), and 2.21 (95% CI, 1.45 to 3.36) (P for trend <0.0001) for systolic BP categories and 1.0, 1.12 (95% CI, 0.82 to 1.52), 1.13 (95% CI, 0.83 to 1.52), 1.30 (95% CI, 0.89 to 1.88), 1.50 (95% CI, 1.01 to 1.88), and 1.54 (95% CI, 0.75 to 3.14) (P for trend=0.026) for diastolic BP categories.

Conclusions—In this large cohort of initially healthy women, BP was strongly associated with incident AF, and systolic BP was a better predictor than diastolic BP. Systolic BP levels within the nonhypertensive range were independently associated with incident AF even after BP changes over time were taken into account.