Backgrounds Athletic heart syndrome (AHS) is a benign condition consisting of physiologic adaptations to the increased cardiac work load of exercise. AHS is associated with a spectrum of alterations in the 12-lead ECG pattern, including marked increase in precordial R-wave or S-wave amplitudes, ST segment changes, and deep Q waves suggestive of left ventricular hypertrophy. These findings may raise the possibility of pathologic condition, but have also been viewed as a consequence of the cardiac remodeling induced by athletic conditioning. However, there has been no report on chronological ECG changes in each individual and their association with exercise capacity. Methods We enrolled 121 healthy young military academic students. They were all male and 20 or 21 years old who showed normal ECG pattern in the 1st year. They had taken regular endurance exercise for 4 years. RR interval, PR interval, QRS duration, deviation of ST segments, amplitudes of Q, R and S waves of standard 12-lead ECG were measured at least once in each year. The running time of 1,500 m was also measured every year to evaluate exercise capacity. In the 4th year, echocardiography was performed to evaluate structural and functional abnormalities. Results Exercise capacity was markedly improved during the 4-year training (1,500 m record in the 1st vs. 4th year: 360짹19 s vs. 337짹17 s , p<0.001). The amplitudes of r��� waves in leads V1, V2 and S waves in V5, V6 were gradually increased (V5 S: 0.38짹0.06 mV in 1st year vs. 0.45짹0.04 mV) and ST segment elevations in the precordial leads were normalized as time ensued (V2 ST segment elevation: 0.31짹0.03 mV in 1st year vs. 0.14짹0.02 mV). Frontal axes were slightly deviated toward right (64짹5쨘 to 77짹6쨘). The amplitude of S in V5 lead was significantly correlated with exercise capacity after training (Pearson coefficient 0.22, p=0.015). There were no structural and functional abnormalities on echocardiography. Conclusion Right ventricular hypertrophy (RVH)-type ECG changes were observed in young healthy individuals who showed increased exercise capacity. These results suggest that RVH could be resulted from long-term endurance exercise.