We have reported previously that asthmatic children have a seasonal variation in the power spectrum in the low frequency, LF (0.04-0.15 Hz) and high frequency, HF (0.15-0.6 Hz) bands of heart rate variability (HRV). In recent years, the l/f fluctuations of heart rate (HR) have been used as a novel index of autonomic function from a chronobiological viewpoint. As asthma is a disease with symptoms affected by seasonal chronobiological changes, we examined whether the l/f fluctuations of HR in asthmatic children changed by season. The subjects examined were 87 asthmatic and 104 healthy children. Using a Holter recorder, a 24-h ambulatory electrocardiogram (AECG) was obtained from each subject. The l/f fluctuations of HR during the time awake, the time asleep, and the 24-h period for each of the four seasons were analyzed by the MemCalc system (Suwa Trust). No seasonal variations of HR l/f fluctuations were detected in either the asthmatic or the control group for any of the three periods analyzed. There was a significant seasonal variation in the mean of the normal-to-normal R-R intervals during the 24-h period in the asthmatic children, but not in the healthy children. In the children with asthma, although their HR varied by season, the l/f fluctuations of HR did not.