In order to elucidate the etiology of the loose shoulder, biochemical characteristics of deltoid and major pectoral muscles and skin collagen from patients with loose shoulder have been investigated. Amounts of soluble collagen, reducible cross-links of collagen and their precursors in both muscles and skin from the loose shoulder were found to be higher than those in the control. These data suggest that soft tissues, especially deltoid muscles, of patients with loose shoulder contain relatively immature collagen fibers compared with normal shoulders. In re-operation cases of the loose shoulder, the collagen fibers in skin were more immature, which may reflect the clinical features of the disease.