The athlete's heart is commonly characterized by an increase in left ventricular mass because of an increase in the left ventricular diastolic cavity dimensions or wall thickness or both. Endurance exercise also induces numerous cardiovascular adaptations, including increased vagal tone. However, the sympathetic function has not yet been precisely elucidated, so the present study evaluated cardiac sympathetic nerve function from metaiodobenzylguanidine (MIBG) images obtained 15 and 180 min after the injection of 123I-MIBG at a dose of 111 MBq. The ratio of heart/mediastinum count (H/M) and the washout rates of 123I-MIBG (WR) were calculated in 25 consecutive patients who were athletes (aged 52+/-13 years) and 23 normal subjects. There was a significant difference in the H/M between the athletic and normal hearts (2.3+/-0.3 vs 2.6+/-0.3, p<0.01, Scheffe's test). An increased WR was observed in the athletes group when compared with the normal group (34+/-4 vs 28+/-3, p<0.01), and there was a significant correlation between WR and the left ventricular mass index (r=0.578, p<0.01). Prolonged exercise training may alter cardiac sympathetic nerve function, which can be detected by MIBG imaging.