Surgical treatment of idiopathic hypertrophic subaortic stenosis (IHSS) was employed in 54 patients; 16 by left ventricular incision (two deaths), 26 by right ventricular incision (five deaths), nine by aortotomy (one death), and four by mitral valve replacement (one early death). The frequent occurrence of mitral regurgitation with IHSS indicates a cause and effect relation between the mitral valve and the left ventricular obstruction. Mitral valve replacement with a low profile floating disk prosthesis (Bjork-Shiley) was successful in relieving the outflow obstruction in these patients and may be a useful method of surgical treatment of this unusual and puzzling cardiac lesion.