Little is known about the structure of athletes' hearts of anatomic variations associated with training. Echocardiograms of 56 active athletes were obtained. Mean left ventricular end-diastolic volume and mass were increased in athletes involved in isotonic exercise, such as swimming (181 ml, 308 g) and running (160 ml, 302 g), compared with controls (101 ml, 211 g); wall thickness was normal (less than or equal to 12 mm). Athletes involved in isometric exercise, such as werstling and shot putting, had normal mean left ventricular end-diastolic volumes (110 ml, 122 ml), but increased wall thickness (13 to 14 mm) and mass (330 g, 348 g). Thus, athletes participating in isotonic exercise had increased left ventricular mass with cardiac changes similar to those in chronic volume overloads. Athletes participating in isometric exercise had increased left ventricular mass with cardiac changes similar to those in chronic pressure loads, recognizing greater left ventricular mass and volume in well-trained athletes aids in interpreting values deviating from "normal" limits.