HCM, as well as coronary and myocardial structural abnormalities, is the most common pathology leading to SCD in young athletes. Furthermore, SCD from fatal arrhythmia seems to be the most common mechanism of death. In this population, however, data are insufficient to support either invasive or noninvasive approaches to clarify risk stratification for SCD. Because of the large population, variants of normal found within the athletic population, and the rarity of the disease, screening for individuals at risk is neither practical nor cost-effective. Not all athletes with HCM are at the same risk for SCD; a thorough history and physical examination should alert the health professional to potential risk factors. Efforts are under way to stratify athletes at risk for SCD to determine who can participate in competitive sports and who should not. However, until research can accurately define variables of hemodynamic and electrical instability that permit reliable identification of athletes with HCM who are at risk for SCD, the recommendation is to disqualify athletes with confirmed HCM from moderate- to high-intensity competitive sports. This recommendation includes athletes with or without symptoms or left ventricular outflow obstruction. Due to the decreased risk of SCD in older athletes, individual judgment of eligibility may be used. Athletes thought to have had myocarditis should be withdrawn from all competitive sports for a convalescent period of approximately 6 months, with thorough cardiac assessment and testing performed before returning to training. Athletes with atrial or ventricular tachyarrhythmia must be screened for structural abnormality, heart response during exercise, and the frequency and duration of the arrhythmia.(ABSTRACT TRUNCATED AT 250 WORDS)