The time-lag of the QT interval adaptation to heart rate changes (QT/RR hysteresis) was studied in 40 healthy subjects (18 females, mean age 30.4±8.1 years) with 3 separate day-time (>13 hour) 12-lead electrocardiograms (ECG) in each subject. In each recording, 330 individual 10-second ECG segments were measured including 100 segments preceded by 2 minutes of heart rate varying >±2bpm. Other segments were preceded by stable heart rate. In segments preceded by variable rate, QT/RR hysteresis was characterized by parameters of exponential decay models. The intra-subject standard deviations (SD) of values were compared with the inter-subject SD of the individual means. The values were also correlated to individually optimized parameters of heart rate correction. Intra-subject SDs of were substantially smaller than the population SD of individual means (0.390±0.197 vs 0.711, p<0.0001). The values were unrelated to the QT/RR correction parameters. Compared to the QTc corrected for averaged RR intervals in 10-second ECGs, and to the averaged RR intervals in 2-minute history, QTc correction for QT/RR hysteresis led to substantially smaller SD of QTc values (11.36±2.00ms, 6.33±1.31ms, and 4.66±0.85ms, respectively, p<0.0001). Thus, the speed with which the QT interval adapts to heart rate changes is highly individual with intra-subject stability and inter-subject variability. QT/RR hysteresis is independent of the static QT/RR relationship and should be considered as a separate physiologic process. Combination of individual heart-rate correction with individual hysteresis correction of the QT interval is likely to lead to substantial improvements of cardiac repolarization studies.