The left ventricle (LV) suffers during various cardiac and extracardiac diseases. After the description of the cardiac performance during human development, the authors present the assessment of the left ventricle function in some cardiac and also extracardiac disorders of the child. There are listed the changes in myocardial structure, systolic and diastolic function in hypertrophy and dilation of the LV determined by systemic diseases that affect the pediatric patients and can lead to severe cardiac failure.