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The formation of true synovial-lined membranes at tissue sites not intimately related to an articulation or a tendon sheath has been described in a variety of pathologic and postsurgical conditions, but until recently has not been well recognized to occur in association with tissue surrounding silicone breast implants. Of 15 cases with resected periprosthetic breast capsules, 7 (47%) demonstrated true synovial metaplasia with capsule-implant interfaces lined by typical synovial cells. Histochemical and immunohistochemical staining reactions were essentially identical to those observed in synovial control cases and featured positive reactions to Alcian blue-periodic acid-Schiff, reticulin, and vimentin. Focal positive immunoreactivity was observed with alpha 1-antitrypsin, alpha 1-antichromotrypsin, lysozyme, and CD68. No immunoreactivity was observed with cytokeratin AE1/AE3, S-100 protein, carcinoembryonic antigen, or basement membrane antigens. Transmission electron microscopy of the lining cells confirmed their true synovial nature with the type A (macrophage-like) cells, type B (fibroblast-like) cells, and intermediate forms or type AB cells identified. We conclude that the cellular lining surrounding silicone breast implants is a true synovial membrane, that synovial metaplasia may occur in nearly one half of all resected periprosthetic capsules, and that awareness of this entity will enable the surgical pathologist to render an accurate histopathologic diagnosis.