The precise composition of calcific deposits in calcifying tendinitis is still unknown. However, analysis of such deposits can help to elucidate the disease's pathogenesis. Twenty-five calcific deposits from various phases of the disease were analyzed by several methods. The macroscopic appearance of the specimens during the acute phase of calcifying tendinitis resembled a milky emulsion; in contrast, it resembled a granular conglomerate during the chronic phase. X-ray diffraction showed a poorly crystallized hydroxyapatite lattice (resembling that in bone) in both phases. Infrared spectroscopy revealed variable H2O, CO3, and PO4 contents in all samples, but no significant differences in these proportions were seen in the two phases of the disease. Organic molecules were seen in addition in all samples. Scanning electron microscopy showed similar morphologies of the crystalline conglomerates of both phases, with somewhat round, nongeometric structures. The macroscopic difference was not reflected in the mineralogic structure. Neither a chemical compositional change nor a change in the crystal lattice was observed. The disintegration of the conglomerates probably depends on a change in the bonding capacity of the organic molecules, which in turn initiates phagocytosis in the resorptive phase.