During a retrospective study, we discovered unusual cardiac manifestations in seven of 21 hyperthyroid patients. Three patients had a transient apical systolic murmur, one had persistent cardiomegaly, and the remaining three had persistent apical systolic murmurs and cardiomegaly; mitral regurgitation and congestive heart failure developed in two of these three patients. Severe, persistent cardiac complications occurred only in patients who had prolonged hyperthyroidism caused by poor compliance with antithyroid drug treatment.