Mitral valve prolapse is the most common cardiovascular abnormality in children and is diagnosed by the presence of a nonejection click with or without an associated murmur. Abnormalities are infrequently seen on ECG, but when present they raise concern as to long-term prognosis. Children who have such abnormalities should undergo exercise testing and Holter monitoring. Patients who have dysrhythmia should be followed closely, even if they are asymptomatic, because they may be at risk for morbidity and death. Restrictions on physical activity are required only if it exacerbates chest pain or dysrhythmia.