In standard textbooks of anatomy it is generally accepted that the fibrous ring of the mitral valve lies in a single plane throughout its entire circumference. Some authors have stated otherwise, but there are no morphometric studies. 30 formalin-fixed hearts were sectioned to reveal the shape of the fibrous ring of the mitral valve and studied by morphometric, radiographic and histological methods. It was noted that the fibrous ring adopted a horizontal “S” shape around the two commissures and the average distance between the highest and lowest parts of the fibrous ring was found to be 7.1 mm (range 6.2–12.5 mm). Radiographic evaluation revealed that the fibrous ring lies approximately in two planes rather than in one. Histological study showed that the fibrous tissue of the mitral valve is continuous with the aortic fibrous ring, and that a slight but significant thickening marks the insertion of the mitral valve. The mitral valve thus lies in two planes, and it is hoped that this information will contribute to cardiac and valvuloplastic surgery. © 1995, Gustav Fischer Verlag Jena GmbH. All rights reserved.