The present study was designed to detect the arrhythmogenic effect of mitral valve prolapse, and the relationship between QT, QT dispersion and ventricular arrhythmias in subjects with mitral valve prolapse. Sixty-four mitral valve prolapse subjects (24 men and 40 women, mean age 27 +/- 6), and 80 healthy control subjects (32 men and 48 women, mean age 28 +/- 7) were studied. The frequency of ventricular arrhythmias by means of 24-h ambulatory electrocardiographic (EGG) monitoring was investigated. Grade greater than or equal to 3 ventricular arrhythmias according to modified Lown and Wolf classification were accepted as complex arrhythmias. The QT intervals were measured from the beginning of depolarization of the QRS complex to the end of the T wave. Using the Bazett formula, QT interval was corrected (QTc) for heart rate. QT dispersion was calculated as the difference between the maximum and minimum QT intervals on any of 12 leads. Premature ventricular complexes seemed to develop in 56 of 64 (87.5%) subjects with mitral valve prolapse. Thirty-eight of the mitral valve prolapse subjects had complex premature ventricular complexes during 24-h ECG and the prevalence of premature ventricular complexes was found to be higher than the control subjects (P < 0.001). We found QT and QTc intervals of 388 +/- 27 and 406 +/- 33 ms in mitral valve prolapse subjects, these values in control subjects were 382 +/- 18 and 402 +/- 11 ms respectively (P > 0.05). QT dispersion and QTc dispersion intervals were 71 +/- 17 and 78 +/- 19 ms in mitral valve prolapse subjects and these values were 43 +/- 16 and 52 +/- 22 ms in control subjects, respectively (P < 0.001). No correlation was found between complex premature ventricular complexes and QT, but there was a correlation between complex premature ventricular complexes and QT dispersion in the mitral valve prolapse subjects. It was concluded that QT dispersion might be a useful marker of cardiovascular morbidity and mortality due to complex ventricular arrhythmias.