Objective. To investigate whether tendon degeneration in posterior tibial tendon dysfunction syndrome is associated with changes in extracellular matrix collagen composition. Methods. Specimens from grossly abnormal tendon regions from 9 patients with posterior tibial tendon dysfunction syndrome were prepared for routine histology. Collagens I, III and V were typed by immunoblotting and quantified by densitometry after SDS-PAGE. Proline and hydroxyproline residues were determined by liquid chromatography. Four other samples from grossly normal homologous tendon regions and one surgical specimen from a healthy patient undergoing arthrodesis of the ankle after an accident were included as control. Results. In the grossly abnormal surgical posterior tibial tendon specimens we observed three types of histopathologic conditions present to varying degrees: increased mucin content, fibroblast hypercellularity and neovascularization. Analysis of degenerate tendons demonstrated a 79.3% increase in total proline and a 32.4% increase in 4-hydroxyproline. In addition, damaged tissue contained a higher proportion of collagen type III (mean increase: 53.6%) associated with a concomitant increase in type V collagen (mean increase: 26.4%). These alterations were accompanied by a reduction in type I collagen (mean decrease: 41.4%). Conclusions. In posterior tibial tendon dysfunction syndrome, the degenerative process results from marked changes in both structural organization and molecular composition of matrix collagens. The higher proportion of type V and type III collagens in degenerated tendons is likely to contribute to a decrease in the mechanical resistance of the tissue.