A teenager with Marfan's syndrome required resuscitation and was found to have multiform premature ventricular contractions and ventricular tachycardia. Of 24 children with Marfan's syndrome, eight (33.3%) were found to have ventricular dysrhythmias, including three with ventricular tachycardia. Six of these eight patients had mitral valve prolapse, and five had prolonged QT or QTU intervals corrected for heart rate. However, only two patients had severe mitral regurgitation, five had only mild heart disease, and one had no detectable heart lesion. The role of mitral valve prolapse and/or delayed repolarization in the development of ventricular dysrhythmia was explored. Delayed repolarization, especially when combined with mitral valve prolapse, is associated with occurrence of ventricular dysrhythmia. Serious ventricular dysrhythmia can occur in children with Marfan's syndrome with or without substantial valve disease, and the dysrhythmia appears to progress with age.