Background Assessment of left ventricular (LV) contractility in mitral regurgitation is difficult, and latent systolic dysfunction may be present despite conventional indexes being in the “normal” range. We sought to demonstrate the presence of latent LV dysfunction in mitral regurgitation by showing diminished contractile reserve with the use of simplified pressure-volume loops from noninvasively derived parameters. Methods and Results We performed exercise echocardiography in 72 patients with isolated mitral regurgitation, minimal or no symptoms, normal resting LV function, and no coronary disease who subsequently underwent uncomplicated valve repair and in 15 healthy subjects. Simplified pressure-volume loops were constructed and LV stroke work was estimated at rest and immediately after exercise. As pressures were estimated, the findings were confirmed with a validated numerical model to estimate stroke work in patients and control subjects. Patients had a higher stroke work than did control subjects at rest (1.45 ± 0.39 vs 0.92 ± 0.21 J, P < .001). Eighteen patients had LV dysfunction develop after surgery. Patients with postrepair dysfunction had a significantly lower exercise stroke work (1.79 ± 0.69) than did patients without (2.28 ± 0.84 J, P = .02), but there was no difference in resting stroke work (1.48 ± 0.39 vs 1.44 ± 0.4, P = not significant). Change in stroke work with exercise was higher in patients with normal postoperative function (0.84 ± 0.59 J) and control subjects (0.59 ± 0.36 J) than patients with postoperative dysfunction (0.31 ± 0.55 J, P = .001). The numerical model confirmed the lower exercise stroke work and change in stroke work in patients with postoperative dysfunction. Conclusions The clinical study and numerical model confirmed diminished contractile reserve in patients who had LV dysfunction develop after uncomplicated mitral valve repair. Diminished contractile reserve may be a preoperative marker of latent contractile dysfunction. (Am Heart J 1999;137:427-34.)