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We evaluated specimens obtained from the biopsy of spontaneously ruptured tendons in 891 patients who were treated between 1968 and 1989. The specimens, which were removed at the time of repair, included 397 Achilles tendons, 302 biceps brachii tendons, forty extensor pollicis longus tendons, eighty-two quadriceps tendons and patellar ligaments, and seventy other tendons. Age and sex-matched control specimens, from 445 tendons taken at the time of death from the cadavera of previously healthy individuals who died accidentally, also were obtained and evaluated. The histopathological analyses of the specimens included light and polarized light microscopy and scanning and transmission electron microscopy. A healthy structure was not seen in any spontaneously ruptured tendon, but two-thirds of the control tendons were structurally healthy (p less than 0.001). There were characteristic histopathological patterns in the spontaneously ruptured tendons. Most (97 per cent) of the pathological changes were degenerative; they included hypoxic degenerative tendinopathy, mucoid degeneration, tendolipomatosis, and calcifying tendinopathy, either alone or in combination. These changes were also found in 34 per cent of the control tendons, but significantly less frequently (p less than 0.001). In the other twenty-six ruptured tendons (3 per cent), the pathological change was an intratendinous foreign body, rheumatoid tendinitis, a xanthoma, a tumor, or a tumor-like lesion such as an intratendinous ganglion. The findings clearly indicate that, at least in an urban population, degenerative changes are common in the tendons of people who are older than thirty-five years and that these changes are associated with spontaneous rupture.