BACKGROUND AND OBJECTIVES: The aim of the study was to evaluate the difference of temporal lability in myocardial repolarization between acute non-Q (NQMI) and Q-wave myocardial infarction (QMI), and to discern whether the locations of myocardial infarction influence such temporal lability. SUBJECTS AND METHODS: Twelve patients with NQMI and 28 with QMI, including 16 anterior (AMI) and 12 inferior MI (IMI) patients were enrolled. Twenty four-hour ambulatory ECG recordings of each patient were analyzed, and the digitized data was partitioned into 30-min sections. The QT intervals were measured using a template matching strategy. We then calculated the low (LF:0.03 - 0.15 Hz) and high frequency (HF:0.15 - 0.4 Hz) power of the QT interval variability using an algorithm capable of removing the influence of the RR-interval on QT interval variability (Normalized Idioventricular QT variability Index:IV-QT). RESULTS: For patients with QMI, the low frequency IV QT (LF IV-QT) was higher than that of NQMI (1.941+/-0.101 and 1.556+/-0.114 respectively, p<0.05). No difference was seen in the high frequency IV QT (HF IV-QT) of the two groups. For QMI patients, both the LF IV-QT and HF IV-QT were higher in day time (6AM-6PM) than in night time (6PM-6AM). Comparing the differences of these indices by the location of QMI, both the LF IV-QT and HF IV-QT of AMI were higher than those of IMI patients (2.231+/-0.135 vs 1.355+/-0.131 and 2.341+/-0.161 vs 1.346+/-0.145 respectively, p<0.0005). Both indices of each group also demonstrated a circadian change. CONCLUSION: In cases of QMI, the temporal lability in myocardial repolarization is larger than that seen in NQMI. Moreover, it was worse in AMI than IMI. Finally, such temporal repolarization lability tends to have a circadian pattern in QMI.