M-mode echocardiography was performed in 54 patients 1 month to 3 years after mitral valvotomy for dominant mitral stenosis. The mitral valve closure index (MVCI), an index previously shown to correlate well with the calculated mitral valve area, was used to diagnose residual mitral stenosis. Using the equation MVCI = 33 MVC--12, 12 patients (22%) were deemed to have mild, 4 (7%) moderate, and 2 (4%) severe residual stenosis. Repeat cardiac catheterisation confirmed the diagnosis in 7 patients. The postoperative mitral valve closure index correlated significantly with patients' subjective symptoms and objective exercise indices. It was higher in patients in whom the posterior mitral leaflet movement became normal after valvotomy, and was lower in patients in whom the anterior mitral leaflet excursions below 14 mm before operation. The mitral valve closure index is a sensitive index for diagnosing residual mitral stenosis after valvotomy.