BACKGROUND In patients with ventricular tachycardia (VT) and apparently normal hearts, mitral valve prolapse (MVP) is discovered fairly often, raising the question of whether or not it is an occasional finding. HYPOTHESIS This issue was analyzed in a series of patients with VT and apparently normal hearts in order to define the prevalence of MVP in this condition, the existence of specific diagnostic features suggesting a nonrandom association between idiopathic VT and MVP, and the prognostic implications of this finding. METHODS We studied 28 consecutive patients with documented VT and no history of heart disease. Two-dimensional (2-D) echocardiogram, cardiac catheterization, morphometric examination of endomyocardial biopsy and arrhythmologic evaluation (24-h Holter monitoring, electrophysiologic study, and signal-averaged electrocardiogram) were performed. Inclusion criteria for all patients were angiographically normal coronary arteries, normal biventricular function, and absence of histologic evidence of myocarditis. Data obtained in patients found to have MVP at 2-D echo were compared with those of the remaining patients. Long-term follow-up data were also collected. RESULTS The prevalence of MVP in our study group was 25% (7 patients). It was not associated with leaflet dysplasia or significant regurgitation. Biventricular function (ventricular volumes and ejection fraction) was comparable in patients with and without MVP. Patients with MVP had a significantly higher prevalence of ventricular late potentials at signal-averaged electrocardiogram (86 vs. 29%, p = 0.027), more interstitial fibrosis at morphometry (8.5 +/- 3.7 vs. 5.4 +/- 2.7% p = 0.028), and VT of right bundle-branch block morphology (100 vs. 48%; p = 0.044). Other arrhythmologic findings were similar in the two groups. After a mean follow-up of > 5 years, no patient in either group died, and none developed heart failure or severe mitral regurgitation. CONCLUSIONS Mitral valve prolapse is frequently detected in idiopathic VT. The distinguishing features of this association are (1) VT of right bundle-branch block morphology, (2) high prevalence of ventricular late potentials, and (3) increased fibrosis on endomyocardial biopsy. Ventricular function and other arrhythmologic findings are not specific of this association. Prognosis remains substantially benign, as is true for most cases of idiopathic VT.