The mitral-aortic intervalvular fibrosa of the heart is of great clinical and surgical importance, because of its involvement in the anatomical and functional integrity of these two valves. In this work, we examined the morphology of the mitralaortic intervalvular fibrosa and its relationship to the mitral and aortic valves. Thirty formaldehyde-fixed adult human hearts of both sexes were dissected and the structural organization, dimensions and area of the mitral-aortic intervalvular fibrosa were determined. The mitral-aortic intervalvular fibrosa was a thin, translucent membranous area located between the root of the aortic artery and the left atrioventricular orifice. In most cases (63%), the mitral-aortic intervalvular fibrosa was approximately triangular in shape, with an area of 93.9 ± 47.4 mm². The lower edge was associated with the anterior cusp of the mitral valve and was 18.0 ± 2.2 mm long, whereas the anterior edge was continuos with the left fibrous trigone and was 10.6 ± 3.0 mm long. The posterior edge was associated with the aortic wall and was continuos with the right fibrous trigone. This edge was 11.7 ± 3.4 mm long. These results may be useful for manufacturing protheses to substitute this injured membrane.