The interrelation between the loudness of the first heart sound, the time interval from the Q wave to the onset of the first heart sound (QM1), and the mitral valve closure rate was studied in nine patients presenting with left atrial myxomata. In seven patients the first heart sound was loud preoperatively and was associated with delayed mitral valve closure. After removal of the myxoma the onset of mitral valve closure returned towards normal, the mitral valve closure rate was reduced, and the first heart sound became softer. In two patients the first heart sound was normal before and after operation as were both the time of onset of mitral valve closure and the mitral valve closure rate. In neither of these patients did the myxoma completely fill the mitral orifice during diastole. The loud first heart sound in left atrial myxoma is a useful clinical sign, and intensity is directly related to the delay in onset of closure of mitral leaflets.