OBJECTIVE: The purpose of this study was to evaluate the circadian rhythm of the autonomic nervous system functions of children with primary monosymptomatic nocturnal enuresis. MATERIAL AND METHODS: The subjects were allocated to three groups--enuresis group (n = 72), normal control group (n = 26), post-treated and cured enuresis group (n = 13). In the three groups. we evaluated the autonomic nervous system function by the power spectrum analysis of heart rate variability (HRV) using 24-hour Holter electrocardiograms. Frequency components of the power spectra of HRV (% power) were divided into three factors; the very low frequency (VLF), the low frequency (LF) and the high frequency (HF) by computer analysis using Fast Fourier Transformation (FFT). RESULTS: In the three groups, the HF component reflecting parasympathetic activity was significantly higher, compared with the LF during sleep. The HF in the enuresis group was significantly higher, compared with the HF in the control group during sleep. The HF in the control group was significantly lower, compared with the LF during waking. However, the HF in the enuresis group was significantly higher, compared with the LF during waking. It showed more increased parasympathetic tone in the enuresis group, compared with that of the control group during sleep and waking. In the post-treated enuresis group, the HF during sleep and waking were significantly decreased, compared with those before treatment. CONCLUSION: These findings suggest the parasympathetic hyperfunction through 24 hours and abnormal circadian rhythm of autonomic nervous system functions in the enuresis group. The post-treated enuresis group showed normal autonomic nervous system functions.