To assess the relationship between left ventricular functional reserve and prognosis in patients with idiopathic mitral valve prolapse, ergometer exercise echocardiography was performed in 10 normal subjects and 30 patients with mitral valve prolapse having either mild, or no mitral regurgitation. These 30 patients with mitral prolapse were followed for 2 to 8 (mean 4.5) years. Increment of % fractional shortening during maximum exercise at the initial study in patients with mitral valve prolaspe and normal subjects were 7 ± 7 and 11 ± 3%, respectively. Based on increment of % fractional shortening, patients with mitral valve prolapse were divided into 2 groups; Group I: 13 cases with Δ% fractional shortening < 5%, Group II: 17 cases with Δ% fractional shortening &ges;5%. The incidence of cardiac symptoms was higher in Group I than in Group II (85 vs 41%, P<0.05). ST-T changes and life-threatening arrhythmias were more frequently observed in Group I. During the follow-up period, M-mode echocardiographic measurements did not vary in Group II, but left ventricular and left atrial dimensions increased significantly (p<0.05, p<0.01, respectively) and % fractional shortening decreased significantly (p<0.01) in Group I without any change in mitral regurgitation severity. Thus, some patients with mitral valve prolapse not having significant mitral regurgitation may develop progressive deterioration of the cardiac function, which may be predicted by exercise echocardiography.