High-angle x-ray diffraction provides direct evidence that amianthoid change, occurring during aging of costal cartilage, corresponds to a transformation from an isotropic to a marked anisotropic distribution of collagen fibrils. Low-angle x-ray diffraction and electron microscopy show that the fibrils have the customary 67-nanometer axial periodicity. Electron microscopy shows that wide amianthoid collagen fibrils consist of smaller parallel fibrils fused together. Similarities between amianthoid change and tendon morphogenesis are briefly discussed. Amianthoid change is remarkable in that aging is accompanied by increased order.