Cross-sectional studies in athletes and untrained subjects suggest that exercise training induces adaptations in cardiac structure and function. However, the role of genetic variation on the results has largely been ignored in these studies. The purpose was to investigate the effects of long-term volitionally increased physical activity on electrocardiographical and echocardiographical parameters in male monozygotic twin pairs discordant for physical activity and fitness. Based on the mailed questionnaires, a phone interview, and the inclusion criteria, 12 pairs of young adult male monozygotic twins were recruited from a Finnish twin cohort. All subjects completed a VO_2max test, electrocardiography and echocardiography studies. Nine pairs had at least 9% difference in VO_2max and were selected for further analysis and to a second echocardiography study. Twins were divided into the more (MAG) and less active group (LAG) according to their VO_2max. On average MAG had 18% higher VO_2max compared to LAG. In electrocardiography MAG had 29% (p=0.02) higher Cornell voltage and 37% (p=0.01) higher right side hypertrophy index. In echocardiography, no significant differences were observed between the groups and LV mass index was only 7% (p=0.16) higher in MAG. These results show that the volitionally increased physical activity that has led to 18% increase in cardiorespiratory fitness induces greater changes in electro- than echocardiographical parameters. Electrocardiographical changes were suggestive of LV hypertrophy and echocardiography showed a similar but statistically non-significant trend.