Gross and histologic examinations of 15 normal cadaver and 15 surgical posterior tibial tendons from patients with posterior tibial tendon insufficiency were performed. All surgical specimens were abnormal with enlargement distal to the medial malleolus and a dull white appearance. At histologic examination, 12 of 15 cadaver tendons displayed normal tendon structure characterized by linear orientation of collagen bundles, normal fibroblast cellularity, low vascular density, and insertional chondroid metaplasia. The surgical specimens displayed a degenerative tendinosis characterized by increased mucin content, fibroblast hypercellularity, chondroid metaplasia, and neovascularization. This resulted in marked disruption of the linear orientation of the collagen bundles.