For the purpose of characterizing the left side of the heart by M-mode echocardiography in former endurance athletes, 13 subjects with a mean age of 66 years, all having competed in elite sports up to the age of 35, were studied. As controls, 21 men, with a mean age of 69 years considered to be sedentary and having no history or signs of heart disease, were examined. The two groups did not differ with regard to heart rate, blood pressure, weight, and height. The mean left atrial dimension (LAD) in the former athletes was significantly larger than in the sedentary controls: 43 +/- 6 mm and 38 +/- 3 mm, respectively (P less than 0.05). This was also true when LAD was corrected for body surface area (BSA): 22.1 +/- 3.3 mm/m2 and 20.0 +/- 1.8 mm/m2 (P less than 0.05). However, no significant differences were seen between the athletes and sedentary controls in the left ventricular end-diastolic dimension and wall thickness. In conclusion, this study indicates that former endurance athletes have an enlarged left atrium but a normal-sized left ventricle.