Electrocardiographic and other abnormalities of highly trained athletes are well recorded but not well enough known, and the more extreme changes still cause diagnostic confusion. This problem was exemplified by 10 athletes, all referred to us by cardiologists because of a diagnosis of organic heart disease. After thorough investigation, including catheter studies, all but 1 of these patients was regarded as normal and all returned to competitive sports. The criteria for diagnosing heart disease in athletes are discussed in light of the findings in 10 patients. Exercise electrocardiography and echocardiography help to exclude organic disease, but left heart catheterization is justified when doubt still exists.