The association of dysrhythmias with mitral valve prolapse (MVP) in pediatric patients was examined by 48-hour ECG monitoring. Fifty unselected children with clinical diagnosis of MVP confirmed by echocardiography were studied. Eight (16%) patients manifested atrial dysrhythmias. Long runs of junctional rhythm with activity were recorded in one patient. Episodes of supraventricular tachycardia were recorded in 4 (8%) children. Seventeen (34%) patients had ventricular dysrhythmias. In 10 (20%) children, ventricular premature complexes (VPCs) were recorded. Four (8%) patients had frequent multifocal VPCs. Ventricular couplets were recorded several times in one patient (2%) and nonsustained ventricular tachycardia was recorded in two (4%) children. Clinical symptoms did not correlate with documented arrythmias. Although the prognostic implications of these findings are uncertain, this study demonstrated that potentially serious dysrhythmias are common in pediatric patients with MVP. It would appear that ambulatory ECG monitoring should be part of the complete evaluation of children with MVP.