Previous studies have shown that a normal LVEF is not a reliable index of LV function in MR. We hypothesized that the forward EF, which is the forward stroke volume (measured by Fick or thermodilution) divided by end-diastolic volume (measured by contrast ventriculography) may be a useful index of LV function, since it represents LV emptying into the aorta. This index was examined in 54 patients with chronic MR who had normal EF (≥50%). There were significant correlations between the forward EF and the end-diastolic volume index (r = -0.69, p < 0.001), end-systolic volume index (r = -0.64, p < 0.001), cardiac index (r = 0.43, p < 0.01), and the ratio of systolic pressure-to-end-syst olic volume (r = 0.65, p < 0.001). Patients were divided into two groups according to the forward EF: group I (n = 34) had forward EF ≤ 35%; and group II (n = 20) had forward EF > 35%. Of the 32 patients who subsequently underwent mitral valve replacement, 24 patients were in group I and eight patients were in group II. At a mean follow-up of 35 months, four patients died; all of them were in group I. Improvement in functional class occurred in 75% of surgical survivors (80% in group I and 63% in group II, p = NS). These preliminary data suggest that forward EF may be a useful index of LV performance in patients with MR who have normal EF. © 1985.