We evaluated the impact of intensive blood pressure control on the incidence of new-onset atrial fibrillation/flutter (AF) and the prognostic implications of preexisting and new-onset AF in SPRINT (Systolic Blood Pressure Intervention Trial) participants. New-onset AF was defined as occurrence of AF in 12-lead electrocardiograms after randomization in participants free of AF at baseline. Poisson regression modeling was used to calculate incident rates of new-onset AF. Multivariable-adjusted Cox proportional hazard models were used to evaluate the risk of adverse cardiovascular events (composite of myocardial infarction, non-myocardial infarction acute coronary syndrome, stroke, heart failure, or cardiovascular death). In 9327 participants, 8.45% had preexisting AF, and 1.65% had new-onset AF. The incidence of new-onset AF was 4.53 per 1000-person years, with similar rates in the standard and intensive treatment arms (4.95 versus 4.11 per 1000-person years; adjusted P=0.14). Participants with preexisting AF (adjusted hazard ratio, 1.83 [95% CI, 1.46-2.31]; P<0.001) and new-onset AF (adjusted hazard ratio, 2.45 [95% CI, 1.58-3.80]; P<0.001) had a greater risk for development of adverse cardiovascular events compared with those with no AF. Participants with preexisting AF who achieved blood pressure <120/80 mm Hg at 3 months continued have a poor prognosis (adjusted hazard ratio, 1.88 [95% CI, 1.32-2.70]; P=0.001) compared with those with no AF. Intensive blood pressure control does not diminish the incidence of new-onset AF in an older, high-risk, nondiabetic population. Both preexisting and new-onset AF have adverse prognostic implications. In participants with preexisting AF, residual cardiovascular risk is evident even with on-treatment blood pressure <120/80 mm Hg. Registration-URL: https://www.clinicaltrials.gov; Unique identifier: NCT01206062.