Our objective was to determine influence of age, body size, and gender on SAECG signals and to establish normal range for standard SAECG parameters in the pediatric population. Five hundred thirty healthy children and adolescents (290 males, 240 females) aged 6.0-17.3 years were studied with high resolution ECG. Patients were divided into five age groups, and each age group was divided in two subgroups according to gender. Parameters studied were filtered QRS duration, root mean square (RMS) voltage of the last 40 ms and of the last 30 ms of the filtered QRS complex, and duration of the terminal waveform of the filtered QRS < 40 microV of amplitude (LAS). After validation of the distribution of the various variables, means, standard deviations, and 95% confidence intervals were determined for each SAECG parameter at 25- to 250-Hz filtering and at 40- to 250-Hz filtering for the ten subgroups. There was a significant age and body surface area (BSA)-associated increase in filtered QRS duration in boys and girls, and a decrease in RMS voltage of the end of the QRS complex in girls. In addition, girls had a significantly shorter QRS and longer LAS durations than boys. These differences of signal-averaged signals between genders were not related to age or body size differences since analysis of covariance with age at evaluation or BSA as covariates showed the same trend. Three children (0.6%) had a SAECG out of normal range for age for the four parameters studied either at 25- or 40-Hz high pass filter. Due to the large sample studied, our results provide a basis for interpretation of SAECG in children. Normal ranges for the various variables were determined, and it appeared that adjustments for age, body size, and gender are mandatory when studying SAECG in children.