We aimed prospectively to determine the incidence of ventricular arrhythmias and ventricular late potentials in children with mitral valvar prolapse, and to assess whether signal-averaged electrocardiography could identify which such children were at high risk of developing ventricular tachycardia. In all, we examined 151 children with mitral valvar prolapse, at an age of 12.2 ± 3.1 years, and 164 healthy subjects aged 12.3 ± 3.7 years. All children underwent 24-hour ambulatory Holter monitoring and echocardiography. The children with mitral valvar prolapse were followed prospectively for a mean of 64 months. There was a significantly higher prevalence of ventricular arrhythmias in those with prolapse than in the controls (p < 0.0001). Runs of ventricular tachycardia were observed in 3 children with mitral valvar prolapse compared with one from the control group. Late potentials were more frequently observed in the children with mitral valvar prolapse than in those who were healthy (p < 0.0001), and also in those with prolapse suffering ventricular arrhythmias compared with those without ventricular arrhythmias (p < 0.02). During follow-up, 24 children with prolapsing mitral valves developed non-sustained ventricular tachycardia, giving a frequency of 3.1/100 subject-years. The sensitivity of late potentials was low, at 52%, for the identification of children with mitral valvar prolapse who developed ventricular tachycardia, although the specificity was high at 90%. This gave a positive predictive value of 50%, and a negative predictive value of 91%. We conclude that prolapse of the mitral valve predisposes to the development of ventricular arrhythmias and late potentials in children. An abnormal signal-averaged electrocardiogram is a specific, but not very sensitive, predictor for the development of ventricular tachycardia in such children.