Ten patients with mechanical pulsus alternans were studied by echocardiography and mechanocardiography. All had been or were in congestive heart failure. An atrial mechanism for pulsus alternans could be identified in two patients: one with primary congestive cardiomyopathy and one after aortic valve replacement for calcific aortic stenosis. Each strong systole was preceded by an "a" wave, while each weak systole was not. This was documented on both the apexcardiogram and the M-mode echocardiogram. Since both patients were in normal sinus rhythm with regular PP intervals, it was concluded that alternating atrial electromechanical dissociation was either the underlying mechanism or contributed to the pulsus alternans. Thus, alternating atrial electromechanical dissociation exists and may cause pulsus alternans. Pulsus alternans is not necessarily the result of left ventricular myocardial dysfunction alone.