OBJECTIVE Previous studies on short-term time series of heart rage suggest an inverse relationship between age and spectral powers of heart rate variability in various frequency bands. In this study, we examined the relationship between age (6-61 years) and long-term heart rate variability. METHODS We obtained 24-h Holter ECG in 33 healthy human subjects (11 children and 22 adults). The heart rate data were analyzed by using spectral analysis and fractal dimensions of the time series. RESULTS We found a significant negative correlation between age and very low frequency (VLF, 0.0033-0.04 Hz), low frequency (LF, 0.04-0.15 Hz) and high frequency (HF, 0.15-0.5 Hz) powers and fractal dimensions during awake as well as sleep periods, and a positive correlation between age and LF/HF ratios. Age and ultra-low frequency (ULF, < 0.0033 Hz) were modestly and negatively correlated only during the awake period. CONCLUSIONS Sleep ULF power is not significantly affected by age, whereas VLF, LF and HF powers and fractal dimensions of heart rate significantly decrease with age during awake as well as sleep periods.