The ultrastructure and mechanical properties of chordae from a surgically removed myxomatous tricuspid valve were examined. Transmission and scanning electron microscopy showed the internal structure of the abnormal chordae to consist of a loose and relatively sparse network of disorganized collagen fibrils, with no well defined collagen bundles present. The abnormal tissue was found to be more extensible than normal and it ruptured at a stress that was only about 6% of the breaking stress for normal chordae. Chordal rupture, a common complication in this disorder, can be attributed to this decrease in chordal strength. At high strains, a decreased elastic modulus was also observed, which may be attributed to tissue changes and an alteration in collagen fibril structure and/or composition.