Abstract : Fibrillatory wave (f-wave) amplitude correlates with left atrium (LA) size in certain electrocardiogram (ECG) leads and it is regarded as a predictor of ablation therapy outcome for atrial fibrillation (AF). This study aims at assessing the temporal stability of f-wave amplitude measures throughout the recording and determining the minimum signal length necessary to characterize them accurately in ECG leads. In a set of standard ECGs acquired in 34 persistent AF patients, we determined the minimum temporal window length W such that the related amplitude value accurately correlated with that from the whole atrial activity (AA) signal in leads I, II, V 1-V 6 (threshold Pearson's correlation coefficient R = 0.9). Subsequently, we tested intrarecord-ing correlation between amplitude values obtained in two distinct W-second AA signal excerpts. This procedure was performed both on the original AA signal and on its principal component analysis (PCA) rank-1 approximation.