Background Increased variability of QT interval ( QTV ) has been linked to arrhythmias in animal experiments and multiple clinical situations. Congenital long QT syndrome ( LQTS ), a pure repolarization disease, may provide important information on the relationship between delayed repolarization and QTV . Methods and Results Twenty‐four‐hour Holter monitor tracings from 78 genotyped congenital LQTS patients (52 females; 51 LQT 1, 23 LQT 2, 2 LQT 5, 2 JLN , 27 symptomatic; age, 35.2±12.3 years) were evaluated with computer‐assisted annotation of RR and QT intervals. Several models of RR ‐ QT relationship were tested in all patients. A model assuming exponential decrease of past RR interval contributions to QT duration with 60‐second time constant provided the best data fit. This model was used to calculate QT c and residual “intrinsic” QTV , which cannot be explained by heart rate change. The intrinsic QTV was higher in patients with long QT c ( r =0.68; P <10 ⁻⁴ ), and in LQT 2 than in LQT 1/5 patients (5.65±1.28 vs 4.46±0.82; P <0.0002). Both QT c and intrinsic QTV were similar in symptomatic and asymptomatic patients (467±52 vs 459±53 ms and 5.10±1.19 vs 4.74±1.09, respectively). Conclusions In LQTS patients, QT interval adaptation to heart rate changes occurs with time constant ≈60 seconds, similar to results reported in control subjects. Intrinsic QTV correlates with the degree of repolarization delay and might reflect action potential instability observed in animal models of LQTS .