The pathogenesis of monosymptomatic nocturnal enuresis is controversial. Various urodynamic studies showed bladder hyperactivity in enuretic children. But the exact cause is not precisely known. The aim of this study was to understand whether the autonomic nervous system dysfunction is involved in this bladder hyperactivity or not. Heart rate variability measurement is widely used for evaluation of cardiac autonomic activity. We evaluated cardiac autonomic nervous system functions in monosymptomatic nocturnal enuretic children by using 24-hour Holter electrocardiogram. The study group consisted of 32 enuretic children (20 boys and 12 girls) and the control group consisted of 20 healthy children (12 boys and 8 girls). In these two groups, we assessed cardiac autonomic regulation by analysis of heart rate variability, and found a significantly higher parasympathetic activity in enuretic children than controls. We suggest that the parasympathetic nervous system hyperactivity plays a role in nocturnal enuresis by causing vesical hyperactivity in monosymptomatic enuretic children. - monosymptomatic nocturnal enuresis; autonomic nervous regulation; heart rate variability; children (C) 2004 Tohoku University Medical Press.