Chronic changes of several structures in around the fetlock tunnel can be a cause of the so-called fetlock tunnel syndrome (FTS) in the horse. Forty-nine annular ligaments (AL) from dead horses without a known history or clinical evidence of lameness and/or digital tendon sheath problems in these legs and 30 AL biopsies from horses suffering from FTS were studied macroscopically and microscopically. Macroscopically, the normal AL had a shiny white appearance, whereas the affected AL were often thicker and less white. Microscopically, the normal AL were about +/- 1 mm thick and were composed of undulating, parallel bundles of collagen. Small blood vessels with a diameter of 0.03-0.12 mm were found. The affected AL showed an increased thickness of collagen bundles, a changed direction of longitudinal axis of collagen bundles, and irregularly dispersed fibroblast nuclei. The number of blood vessels had increased, the external diameter of arteriolae could be up to 0.3 mm and arterial wall changes were observed. Possible relationships between the histological findings and the aetiology of the FTS are discussed.