BACKGROUND The aim of the present study was to assess the prevalence and the clinical significance of ventricular late potentials in mitral valve prolapse. METHODS Two hundred subjects (126 women and 74 men) with mitral valve prolapse, and a mean age of 37 +/- 17 years, were studied. Fifty eight per cent of them exhibited signs of mitral regurgitation at Doppler analysis of mitral flow. A 24-hour Holter recording and a signal-averaged electrocardiogram were performed in all the patients. Late potentials were considered present if the filtered QRS complex was > 110 ms and the root-mean-square amplitude in the last 40 ms of the filtered QRS was < 25 microV after 25 Hz filtering and respectively > 114 ms and < 20 microV after 40 Hz filtering. The duration of low-amplitude signals < 40 mV had to be > 38 ms. RESULTS Ventricular late potentials were detected in 45 patients (22.5%) and were unrelated to subjects age, sex and electrocardiographic ST-T changes. Fourty one per cent of the subjects exhibited lown class > or = 3 ventricular extrasystoles at Holter ECG monitoring, while the remaining subjects (59%) had Lown classes < 3 ventricular arrhythmias. Complex ventricular arrhythmias were more common in the subjects who exhibited late potentials (55.5%) than in the remaining population (36.7%) (p < 0.03). A higher frequency of late potentials was found in the patients with mitral regurgitation (34.7%) than in those without (5.8%) (p < 0.0001). In the 55 subjects who manifested mitral regurgitation and complex ventricular arrhythmias, the prevalence of late potentials was 43.6%, while in the 61 without complex arrhythmias the prevalence was 26.6% (p = 0.05). CONCLUSIONS In subjects with mitral valve prolapse the signal-averaged electrocardiogram allows to identify a subgroup of patients with more serious ventricular arrhythmias. Mitral regurgitation seems to be the main determinant of the arrhythmogenic substrate present in these patients.