Phonocardiographic clues to the diagnosis of left atrial myxoma based on 3 cases are presented. Vibrations during the isovolumic period following a first heart sound of increased amplitude, along with simultaneous vibrations in the systolic upstroke of the apex cardiogram, are clues to the presence of a left atrial myxoma. These vibrations probably are associated with its movement (sudden acceleration and deceleration of the tumor and surrounding blood). Similar vibrations may be recorded phonocardiographically during the isovolumic relaxation period. The presence of a long Q-S1 or short A2-EDS (early diastolic sound) interval is not helpful in differentiating left atrial myxoma from mitral stenosis. When the clinical findings suggest severe mitral stenosis, a long A2-EDS should raise suspicion of a left atrial myxoma. Systolic, diastolic and presystolic murmurs may occur either in mitral stenosis or with left atrial myxoma and are not of themselves helpful in differentiation, unless there is unusual temporal or postural variation, or both, in these murmurs. © 1969.