The authors examined 119 tendons with light-, and 34 tendons with electron microscope, excised within 48 h after spontaneous rupture of tendon. By light microscopic study 9, and by electron microscopic examination 18 cases of calcifying tendinopathy could be detected. The calcification occurred without necrosis or inflammation. The authors concluded, that calcifying tendinopathy is a hypoxic alteration of the tendon.