We evaluated biopsy specimens from the Achilles tendon in 40 patients with long-standing achillodynia and an ultrasonographic widened tendon with hypoechogenic areas. We used a standardized protocol to assess the general tendon pathology score of paraffin-embedded specimens stained with HE. Stereologic measurement of the volume density of glycosaminoglycan (GAG)-rich areas, stained with the Alcian blue (pH 2.5)/periodic acid Schiff method (AB/PAS) was performed. 14 specimens obtained at autopsy served as reference material. Abnormal fiber structure and arrangement, focal variations in cellularity, rounded nuclei, decreased collagen stainability and increased non-collagenous extracellular matrix were seen in all biopsy specimens. Slight histopathological changes were noted in half of the controls. Increased vascularity was present in two thirds of the patient specimens and in one third of the controls, and signs of perivascular hemorrhage, as evidenced by hemosiderin deposition in 6/40 of the patients, but in none of the controls. The volume density of GAG-rich areas was higher in the patients 0.47 (0-0.86) than in the controls 0 (0-0.07). Changes in the fiber structure and arrangement, as well as increased amounts of interfibrillar GAG, appear to be characteristic morphological features in Achilles tendons with long-standing achillodynia and ultrasonographic widening. These findings may indicate that achillodynia is due to local disturbances in connective tissue metabolism or circulation or to both.