Objectives: This study was designed to evaluate whether different antihypertensive treatment regimens with similar blood pressure reduction have different effects on new-onset atrial fibrillation (AF). Background: It is unknown whether angiotensin II receptor blockade is better than beta-blockade in preventing new-onset AF. Methods: In the Losartan Intervention For Endpoint reduction in hypertension (LIFE) study 9,193 hypertensive patients and patients with electrocardiogram-documented left ventricular hypertrophy were randomized to once-daily losartan- or atenolol-based antihypertensive therapy. Electrocardiograms were Minnesota coded centrally, and 8,851 patients without AF by electrocardiogram or history, who were thus at risk of developing AF, were followed for 4.8 ± 1.0 years. Results: New-onset AF occurred in 150 patients randomized to losartan versus 221 to atenolol (6.8 vs. 10.1 per 1,000 person-years; relative risk 0.67, 95% confidence interval [CI] 0.55 to 0.83, p < 0.001) despite similar blood pressure reduction. Patients receiving losartan tended to stay in sinus rhythm longer (1,809 ± 225 vs. 1,709 ± 254 days from baseline, p = 0.057) than those receiving atenolol. Moreover, patients with new-onset AF had two-, three- and fivefold increased rates, respectively, of cardiovascular events, stroke, and hospitalization for heart failure. There were fewer composite end points (n = 31 vs. 51, hazard ratio = 0.60, 95% CI 0.38 to 0.94, p = 0.03) and strokes (n = 19 vs. 38, hazard ratio = 0.49, 95% CI 0.29 to 0.86, p = 0.01) in patients who developed new-onset AF in the losartan compared to the atenolol treatment arm of the study. Furthermore, Cox regression analysis showed that losartan (21% risk reduction) and new-onset AF both independently predicted stroke even when adjusting for traditional risk factors. Conclusions: Our novel finding is that new-onset AF and associated stroke were significantly reduced by losartan- compared to atenolol-based antihypertensive treatment with similar blood pressure reduction.