Heart rate variability (HRV) has become the conventionally accepted term for describing variations in both instantaneous heart rate and R-R intervals. In the pediatric age group, HRV has been investigated in healthy children, diabetics, respiratory distress syndrome of the newborn, and sudden infant death syndrome. This study aimed to evaluate HRV in pediatric patients with dilated cardiomyopathy and to compare it with that of age-matched normal subjects. The study evaluated 21 patients with dilated cardiomyopathy: 11 females (mean age, 7 ± 4 years; range, 2-17 years) and 10 males (mean age, 10 ± 6 years; range, 2-18 years). Dilated cardiomyopathy (DCM) was diagnosed according to commonly accepted criteria after a noninvasive cardiologic examination, echocardiography, and 24-h Holter monitoring (MR45 and MR45-3 Oxford recorder). The patients were divided into six groups according to age, sex, and type of cardiomyopathy. Heart rate variability was recorded and analyzed in the time domain. The patients with DCM showed an abnormal HRV pattern. Particularly in the 5-6-year-old male patient group, the HRV values all were significantly increased (p = 0.05). In the 2-6-year-old female patient group, the mean cycle length, the standard deviation of all normal sinus R-R intervals during 24 h (SDNN), and the standard deviation of the average normal sinus R-R intervals for all 5-min segments (SDANN) were significantly increased (p = 0.05). The 13-18-year-old female patient group showed a significant reduction in SDNN and the mean of the standard deviation of all normal sinus R-R intervals for all 5-min segments (SDNNi) (p = 0.05). The modification of the HRV pattern in the time domain, partially age- and gender-dependent modification, may reflect an imbalance of the autonomic nervous system in children who show a delayed or reduced activity, such as pediatric patients with DCM.