Obesity has become a major health problem worldwide. Cardiovascular abnormalities have been described not only in obese adults but also in obese children and adolescents. The aim of the present study was to detect early left ventricular structural and functional changes in obese Egyptian children and adolescents without comorbidities. Anthropometric, laboratory and echocardiographic parameters were obtained from 50 obese children and adolescents with an average body mass index (BMI) of 32.7 ± 5.2 and compared to 25 non-obese sex- and age-matched controls, with a BMI of 17.4 ± 3.2. Cardiac dimensions, left ventricular systolic and diastolic functions were evaluated. The obese group had a significantly higher end-diastolic septal ((0.90 ± 0.17 vs 0.62 ± 0.06, p < 0.01) and posterior wall thickness (0.72 ± 0.14 vs 0.57 ± 0.06, p < 0.01) and left ventricular mass index (47.40 ± 25.62 vs 28.16 ± 3.12, p < 0.01) than the non-obese group. BMI and waist circumference values showed significant correlations with these echocardiographic variables. Obese children showed abnormal diastolic parameters whereas systolic function remained normal. Multiple regression analysis showed that the body mass index is a significant predictor of left ventricular mass index. Obese children and adolescents have significant left ventricular hypertrophy and changes in early diastolic filling, even in the absence of other comorbidities, early in the course of the disease.