We report the histopathologic findings in two cases of idiopathic calcifying tenosynovitis, one involving the insertion of the right pectoralis minor tendon, and the other the left ring finger proximal interphalangeal joint of a 54-year-old male and a 28-year-old female, respectively. At operation, cream cheese-like material was identified in each case. Sections of tendon in the first case showed numerous circumscribed lesions in various stages of evolution. Early lesions consisted of central granular or globular proteinaceous material surrounded by histiocytes, lymphocytes, arid foreign-body giant cells, with a peripheral network of small blood vessels. Larger, more mature lesions contained similar proteinaceous material, but had central cavitation, a thinner reactive inflammatory zone, and a fibrous capsule. The largest lesions were cystic, had no proteinaceous material or significant inflammatory zone, and had a thick fibrous capsule. No calcification was identified in the intratendinous lesions. In contrast, the hyperplastic synovium and other peritendinous tissues contained numerous round psam-moma-like calcifications. It is concluded that the synovial and peritendinous psammoma-like calcifications in our cases represent a reaction to a primary tendinous lesion, which may be a consequence of ischemia or persistent mild trauma.