Background: Although low atrial septal (LAS) pacing may prevent atrial tachyarrhythmias in selected patients, far-field R-wave (FFRW) sensing in this region seems more likely than in the right atrial appendage. Methods and Results: We compared the clinical characteristics and prevalence of FFRW sensing in 31 recipients (mean age, 74±10 years) of dual-chamber pacemakers, randomly assigned to 10.0mm (n=15) vs. 1.1mm (n=16) tip-ring electrode spacing of bipolar atrial leads implanted in the LAS for management of bradyarrhythmias. The pacemakers were programmed to DDD mode with backup rates at 50-60beats/min. FFRW sensing was measured with atrial sensitivity set at 0.1mV, and increased in 0.1mV steps. Predictors of FFRW sensing were examined by multiple variable regression analysis, and hazard ratios (HR) and confidence intervals (CI) were calculated. At atrial sensitivities of 0.1 and 0.5mV, FFRW was sensed in 24 (77%) and 9 (29%) patients, respectively. A 10.0-mm tip-ring electrode spacing of the atrial lead (HR 10.3; 95%CI 1.0-102.7; P=0.047), and presence of left ventricular hypertrophy (LVH) on 12-lead ECG (HR 14.5, 95%CI 1.2-180.0; P=0.037) were independent predictors of FFRW sensing. Conclusions: The prevalence of FFRW sensing in the LAS region was high. A narrow spacing of the tip-ring electrodes is recommended in the LAS, particularly in the presence of LVH on ECG. (Circ J 2012; 76: 598-606)