Eleven members of a family with a high prevalence of mitral valve prolapse were investigated. Seven had documented sinus bradycardia, and five had mitral valve prolapse. Three patients with both mitral valve prolapse and bradycardia had recurrent syncope reproduced by simple head up tilting, and in one patient this resulted in asystole. The hemodynamic responses to isoproterenol and phenylephrine administration were normal. Supine plasma norepinephrine levels were normal in all three and increased appropriately in two of three patients after tilting. Atrial pacing studies documented marked prolongation of atrial His intervals and inability to maintain 1:1 atrioventricular conduction when paced at a rate of 120/min. These findings were reversed by atropine. This family shows a close correlation between mitral valve prolapse and potentially lethal bradycardia. Excessive vagal tone is believed to be responsible for both bradycardia and sinus arrest, which in two patients was prevented by permanent demand pacing.