BACKGROUND In spite of growing interest in the problem of left ventricular hypertrophy in children there is relatively little published data on normal values of the left ventricular mass (LVM) among data on normal values in the developmental age. OBJECTIVE To determine the normal values of LVM in infants and children in relation to body surface area (BSA), age (in months) and gender. METHODS This study was carried out on 773 children (403 girls and 370 boys, aged from 2 weeks to 18 yrs). They were referred for echocardiographic investigations from paediatric and neonatal hospital departments, from outpatients clinics and primary schools. In all patients cardiovascular disease was excluded. LVM (in grams) was calculated in relation to BSA, age in months and sex. All echocardiographic studies were carried out by the same investigator, according to the American Society of Echocardiography (ASE) convention, modification 2D using, Philips HD14000 Ultrasonograph. The LVM values were calculated at the end of diastole from: left ventricle dimension (LVDd), interventricular septum thickness (IVSd) and thickness of the posterior wall of the left ventricle (LVPWd). Data were analysed in relation to the body surface area (in 17 consecutive intervals from 0.2 m(2) to 1.8 m(2)) and to age (in months) for all children and for girls and boys separately. LVM values were calculated as a mean +/- SD and extreme values 5 or 95 percentile. The results were accepted as normal when they did not exceed 2SD or 5(th) or 9(th) percentile. Statistic analysis was performed with the use of rank correlation Spearman test and Mann-Whitney U.test. RESULTS The left heart ventricular mass was determined on the base of data from healty children aged 2 weeks to 18 years. A significant correlation of LVM with BSA and with age was found. There were no significant gender dependent differences in LVM values. Abnormal values exceeding the 95(th) percentile were present in 25 (3.2 %) examined children. CONCLUSIONS 1. LVM values obtained in this study can be used as reference values. 2. No gender differences were found in LVM. 3. BSA is a good reference parameter in calculating LVM in children aged 2 weeks-18 years. Both BSA and age give high correlation with left ventricular hypertrophy, in children with left muscular ventricular mass exceeding 95 percentile or + 2SD, long term care in department of cardiology is recommended.