therapies for atrial tachyarrhythmias(ATs) by measuring the cumulative time(burden) the patient spends in arrhythmia. Contradictory results questioned either therapy efficacy or statistical power of the trials. We studied AT burden variability in patients paced for sinus node disease(SND) in order to interpret currently published data appropriately and to evaluate reliable sample sizes. Methods and results: One hundred and five patients with AT and SND received a dual chamber pacemaker with antitachyarrhythmia- pacing capability, and were followed for 13 months. Seventy- eight patients(74% ) suffered AT recurrences. Device- gathered diagnostic measures were used to simulate results of randomized studies both with crossover and parallel design. The sample size required for statistically significant results was calculated as a function of the expected therapy- induced burden reduction. AT burden intra- patient variability was high: 43% of patients showed intrinsic fluctuations hiding any therapy- induced burden reduction lower than 30% . Demonstrating therapeutic breakthrough through a 6 month study would require 290 patients with crossover design and 5800 patients with parallel design. Doubling the study period requires 400 and 3000 patients, respectively. Conclusion: Patients with AT and paced for SND showed high intra- patient burden variability, which could possibly hide an AT burden reduction induced by a therapy. Previous studies involving non- pharmacological therapies utilizing AT burden end- points could lack the power to reach statistical significance. therapies for atrial tachyarrhythmias (ATs) by measuring the cumulative time (burden) the patient spends in arrhythmia. Contradictory results questioned either therapy efficacy or statistical power of the trials. We studied AT burden variability in patients paced for sinus node disease (SND) in order to interpret currently published data suitably and to evaluate reliable sample sizes. Methods and results: One hundred and five patients with AT and SND received a dual chamber pacemaker with antitachyarrhythmia-pacing capability, and were followed for 13 months. Seventy-eight patients ( 74%) suffered AT recurrences. Device-gathered diagnostic measures were used to simulate results of randomized studies both with crossover and parallel design. The sample size required for significant significant results was calculated as a function of the expected therapy-induced burden reduction. AT burden intra-patient variability was high: 43% of patients saw intrinsic fluctuations hiding any therapy-induced burden breakthrough lower than 30%. Demonstrating therapeutic breakthrough through a 6 month study would require 290 patients with crossover design and 5800 patients with parallel design. Doubling the study period requires 400 and 3000 patients, respectively. Conclusion: Patients with AT and paced for SND showed high intra-patient burden variability, which could possibly hide an AT burden reduction induced by a therapy. Previous studies involving non- pharmacological therapies utilizing AT burden end- points could lack the power to reach statistical significance.