Aims: Atrial fibrillation (AF) is associated with increased cardiovascular risk and the incidence increases with age, hypertension and left ventricular hypertrophy (LVH). Reducing in-treatment systolic blood pressure (SBP) prevents new-onset AF but has previously not been studied in patients with isolated systolic hypertension (ISH). We aimed to investigate the effect on preventing new-onset AF by decreased in-treatment SBP in patients with ISH compared to patients with non-ISH. Methods and results: Double-blind, randomized, parallel-group study of 1320 patients with ISH and electrocardiographic (ECG) LVH, included among the 9193 patients in the Losartan Intervention For Endpoint reduction in hypertension (LIFE) study. Annual ECGs were Minnesota coded centrally, and new-onset AF was evaluated in 1248 ISH patients and compared with 7583 non-ISH patients during mean 4.8 ± 0.9 years follow-up. Cox regression analyses were used to assess the effect of reduced in-treatment SBP. New-onset AF occurred in 61 (4.9%) ISH patients and 292 (3.9%) non-ISH patients. In multivariate analysis lower in-treatment SBP was associated with 17% risk reduction (p = 0.008) for new-onset AF in ISH patients and 9% risk reduction (p = 0.006) in non-ISH patients per 10 mmHg decrease in in-treatment SBP, independent of treatment modality, baseline risk factors, baseline SBP and in-treatment heart rate and ECG-LVH. There was a significant interaction (p = 0.041) in favor of SBP reduction and AF prevention in ISH vs. non-ISH patients. Conclusion: Our data suggest that the effect of in-treatment SBP reduction in preventing new-onset AF is stronger in ISH compared to non-ISH patients with hypertension and ECG-LVH. However, the principal findings were the same in ISH and non-ISH patients.