Paroxysmal supraventricular tachycardia (PSVT) is associated with altered hemodynamics. We describe the echocardiographic features of myocardial dysfunction during a prolonged episode of PSVT in an 11-year-old male. The abnormality of the phases of cardiac activity presented as a markedly prolonged left ventricular systole (320 msec) and isovolumic relaxation phase (220 msec) and a shortened and delayed diastolic filling period (140 msec). These abnormalities reverted to normal immediately after spontaneous conversion to sinus rhythm. Propranolol, which was used to prevent PSVT in this child, may have been involved in the mechanism of the altered mechanical events.