Calcified substances from lesions of calcified tendinitis of the shoulder (CTS) were studied by scanning electron microscopy (SEM) and polarized light microscopy. The substances were also analysed by infra-red spectrography. The rotator cuffs of the shoulder in 9 cadavers were observed by light microscopy. In the histological study of the rotator cuffs, various degenerative changes, such as separation and irregular arrangement of the collagen bundles and hyalinization at the attachment of the rotator cuff tendon to the bone were found. Calcification at the tendon insertion was also detected at the blue line area, which was stained with hematoxylin, while the musculo-tendinous juntion was intact. Scanning electron micrographs showed that calcified deposits in the CTS lesions were made up of granular crystals (0.1-0.2μ in diameter) accumulating into round particles (3-10μ in diameter). Infra-red spectrography demonstrated that the deposits were similar to commercially available hydroxyapatite. Calcification can be induced by aging and mechanical stresses which proceed degenerative changes and circulatory disturbances at the tendon insertion. The calcified crystals, composed of hydroxyapatite, can push into the subacromial bursa and cause crystal-induced synovitis.