OBJECTIVES: Most researchers use the time domain and spectral analysis in the assessment of heart rate variability (HRV), while others use either the time or frequency domain measures. In this study, we investigated the presence of correlation between the time and frequency domain indices of HRV in normal healthy children and in patients with atrial septal defect (ASD). STUDY DESIGN: A total of 60 children, 28 with ASD and 32 healthy children, were recruited. Time domain measures and frequency domain measures were analyzed from the 24-hour Holter ECG records. Correlation between time domain measures and frequency domain measures as well as correlation within the time domain measures was computed in each group. RESULTS: There was a positive correlation among all the measurements except the low- (LF) and high- (HF) frequency (LF/HF) ratio which was negatively correlated. The degree of correlation was stronger in some variables and weak in others. CONCLUSION: We have shown that time domain measures are correlated with frequency domain measures in both ASD patients and in healthy children. Some of these indices are so strongly correlated with each other that they can be used interchangeably.