An increase of coronary artery size after endurance training has been suggested by experimental data on animals and anecdotal autopsy reports in men. However, systematic studies on in vivo coronary anatomy of athletes have been lacking so far. We utilized two-dimensional echocardiography (2DE) to explore non-invasively the coronary anatomy of endurance athletes. Twenty long-distance runners (LDR) and 20 matched sedentary controls (SC) were studied initially. Visualization of the ostia and main trunks of-right (RCA) and left coronary artery (LCA) was obtained in 90-100 p. 100 of LDR and 70-75 p, 100 of SC. Collateral branches of LCA (anterior descending artery and circumflex branch) and RCA were visualized respectively in 60-70 p. 100 and 30-40 p, 100 of cases. The very good quality of images made possible the measurement of LCA and RCA size. LDR as a group had significantly larger coronary arteries than SC: this was associated with significant left and right ventricular enlargement and hypertrophy. These results have been further confirmed in a large survey of triathletes, swimmers, water-polo players, young LDR and prepubescent football players. Increase of coronary artery size is a well-documented effect of endurance training which can be easily investigated with 2DE, However, the large interindividual variability and the observation of very large coronary arteries in adolescent subjects suggest that genetic factors may also play a role in determining the final size of the coronary vessels.