Repairing mitral regurgitation (MR) requires an understanding of its mechanism. Evaluating restricted leaflet closure in functional MR is challenging. Tenting area between leaflets and annulus in long-axis (LAX) views correlates with MR, but is positive even in control subjects; in the 4-chamber view, the incomplete mitral leaflet closure (IMLC) tenting pattern may be subtle and variable. We tested the hypothesis that leaflet concavity toward the left atrium in the LAX view, a rapid visual clue indicating abnormal tethering predominantly by intermediate chords, is a strong indicator of functional MR. We reviewed 90 patients: 40 with inferior myocardial infarction and ejection fraction > or = 50%; 40 with global left ventricular dysfunction and ejection fraction < 50%; and 10 control subjects. We assessed leaflet shape (concave or convex toward the left atrium) and maximum systolic proximal MR jet width in the LAX views. To quantify shape, we measured the leaflet concavity area between the anterior leaflet and a line connecting its ends. Conventional IMLC area was also assessed. Patients with leaflet concavity had significantly greater MR than those without this finding (jet width of 4.6 +/- 0.7 vs 0.5 +/- 0.1 mm, P <.0001), indicating mild-moderate versus trace MR, with differences comparable in those with inferior myocardial infarction and left ventricular dysfunction. Leaflet concavity area most strongly predicted MR by multivariate regression (R(2) = 0.7). Conventional IMLC area did not uniquely distinguish patients with or without MR and correlated more weakly with MR (R(2) = 0.30 vs 0.73). Mitral leaflet concavity in the LAX view provides rapid and reliable recognition of functional MR, with greater reliability than IMLC area. This shape, consistent with tethering by intermediate chords, may have implications for potential intervention.