To assess the degree of mitral subvalvular fibrosis (SVF) in patients with mitral stenosis, left ventriculograms were reviewed, and a mitral subvalvular distance ratio (DR) was determined by dividing the distance from the papillary muscle tips to the closed mitral leaflets in systole by the distance from the aortic valve to the left ventricular apex in diastole. In 15 patients with normal ventricles, the mean DR was 0.244 ± 0.017. In nine patients with minimal SVF, the mean DR was 0.181 ± 0.025. In 10 patients with moderate SVF, the mean DR was 0.148 ± 0.033. In five patients with severe SVF, the mean DR was 0.087 ± 0.009. All mean DRs were mutually distinguishable at p <0.01. The clinical results and preoperative DR of 28 patients who had mitral commissurotomy were reviewed. A DR of <0.140 precluded a good long-term result from mitral commissurotomy alone. The DR reliably predicts the degree of SVF preoperatively and identifies patients with mitral stenosis who require valve replacement or a direct surgical attack on the sub-valvular mechanism.