Object: Quantitative T-2 measurement is applied in cardiac Magnetic Resonance Imaging (MRI) for the diagnosis and follow-up of myocardial pathologies. Standard Electrocardiogram (ECG)-gated fast spin echo pulse sequences can be used clinically for T-2 assessment, with multiple breath-holds. However, heart rate is subject to physiological variability, which causes repetition time variations and affects the recovery of longitudinal magnetization between TR periods. Materials and methods: The bias caused by heart rate variability on quantitative T-2 measurements is evaluated for fast spin echo pulse sequence. Its retrospective correction based on an effective TR is proposed. Heart rate variations during breath-holds are provided by the ECG recordings from healthy volunteers. T-2 measurements were performed on a phantom with known T-2 values, by synchronizing the sequence with the recorded ECG. Cardiac T-2 measurements were performed twice on six volunteers. The impact of T-1 on T-2 is also studied. Results: Maximum error in T-2 is 26% for phantoms and 18% for myocardial measurement. It is reduced by the proposed compensation method to 20% for phantoms and 10% for in vivo measurements. Only approximate knowledge of T-1 is needed for T-2 correction. Conclusion: Heart rate variability may cause a bias in T-2 measurement with ECG-gated FSE. It needs to be taken into account to avoid a misleading diagnosis from the measurements.