Imbibition analysis, a polarised light microscopy technique, was used to examine the molecular organisation of collagen in normal and diseased mitral valve chordae tendineae. A single strut chorda from each of 23 valves (14 from necropsy specimens and nine from valve replacement surgery) was studied. The degree of molecular organisation of collagen in unstained 7 micron sections of the chordae was assessed by measuring the retardation of polarised light by the sample. Sections from each tendon were examined, after staining with Movat's pentachrome, for the presence of proteoglycan infiltration and classified as normal or abnormal on that basis. The imbibition analysis results were grouped accordingly. The retardation in the collagen in the seven chordae with proteoglycan infiltration was significantly lower than in the 16 normal chordae, indicating decreased molecular organisation. Five of the seven abnormal chordae with proteoglycan infiltration and decreased retardation were from patients with floppy mitral valves; the other two were from normal necropsy specimens. Although proteoglycan infiltration may not be a specific marker for floppy valve disease, its presence is associated with decreased molecular organisation of collagen in the chordae. Degradation of the ground substance bound to the collagen is the most plausible explanation for the measured optical changes.