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Although electrocardiography (ECG) is recommended in all subjects with hypertension, no information is available on the influence exerted by random changes in the placement of electrodes on the day-to-day variability of ECG criteria for diagnosis of left ventricular hypertrophy (LVH).In a multicentre, randomized study, two standard 12-lead ECG were recorded, 24 h apart, from 276 consecutive hypertensive patients (mean age 65 ± 12 years, 49.6% men). Overall, 142 patients were randomized to ECG with the position of electrodes marked on the skin using a dermographic pen and 134 to traditional ECG without marking the position of electrodes. Day-to-day variability of ECG criteria for LVH was compared between the two groups.Coefficients of variation (SD of the difference between paired voltage measurements divided by the mean value) varied consistently among subjects randomized to ECG without dermographic pen, ranging from 30% (R wave in lead I) to 81% (R wave in lead V5). Dermographic pen led to a lesser variability of ECG voltages with consequent reduction in the coefficients of variation, which ranged from 26% (R-wave amplitude in lead I) to 43% (R-wave amplitude in lead V5). The proportion of subjects who changed classification status for LVH (‘reclassification rate’) from the first to the second ECG session (LVH present in session 1 and absent in session 2, or vice versa) decreased for effect of dermographic pen from 11 to 4% (P = 0.040) with the Cornell voltage, from 19 to 11% (P = 0.029) with the Sokolow–Lyon voltage, and from 18 to 7% with the Romhilt–Estes criterion (P = 0.018), but not with other criteria. In particular, the typical strain and the Cornell strain were associated with the lowest reclassification rates regardless of dermographic pen.Random changes in the position of ECG electrodes strongly impair the day-to-day reproducibility of Cornell voltage, Sokolow–Lyon and Romhilt–Estes criteria for LVH. The typical strain and Cornell strain criteria showed a lesser spontaneous day-to-day variability.