An association between mitral valve prolapse (MVP) and hyperthyroidism has been described in adults. However, the long-term prognosis when associated with significant mitral regurgitation remains unclear. Three consecutive children with Graves' disease were found to have a loud mitral regurgitation murmur (grade III/VI) and echocardiographic evidence of MVP with moderate mitral regurgitation. The cardiac manifestations included palpitations, exertional dyspnea, and exercise intolerance. The electrocardiograms at presentation were sinus tachycardia in all cases. All had hyperthyroidism and positive thyroid antibodies. Exophthalmos occurred in two and appeared later than the cardiac symptoms in one. The cardiac murmur disappeared in all patients when antithyroid agents controlled the hyperthyroidism. Follow-up echocardiography showed normal in one and MVP with mild mitral regurgitation in two. We conclude that MVP and significant mitral regurgitation can occur in children with hyperthyroidism, especially those with Graves' disease. The prognosis is good after adequate medical control of the hyperthyroidism.