Aim. To study the electrocardiographic characteristics of secondary atrial septal defect (ASD) depending on its size. Subjects and methods. A total of 80 patients (32 men and 48 women; mean age 23.5±2.7 years) with this defect were examined and divided into three groups according to its size relative to body surface area. Results. The degree of electrocardiographic changes in secondary ASD depended on its size: a significant positive correlation was established between defect size and P wave amplitude (r=0.8; p<0.005), as well as its duration (r=0.5; p<0.001). A positive correlation was noted between defect size and R wave amplitude in lead V1 (r=0.6; p<0.04), that and S wave amplitude in lead V5 (r=0.3; p<0.02); between the size of the defect and the degree of an increase in the total index RV1+SV5,V6 (r=0.7; p<0.02); and that and the degree of electrical heart axis (EHA) deviation to the right (r=0.3; p<0.03). Conclusion. The degree of electrocardiographic changes in secondary ASD depends on its size. Giant defects are characterized by a frequent EHA deviation to the right, pronounced signs of right ventricular hypertrophy, and a higher prevalence of right bundle branch block.