The present study reports on the fine structure of human costal cartilage at different ages in order to obtain information on the morphogenesis of amianthoid fibers. Our results reveal an overall increase of collagen fibril diameter with increasing age, even in areas with no signs of amianthoid transformation. Ultrastructural evidence is presented that this increase in diameter is due to a gathering of the preexisting collagen fibrils. The age-related change in collagen fibril diameter is paralleled by changes in the composition and ultrastructural appearance of cartilage proteoglycans (as revealed by acridine orange staining). Acridine-orange-positive filaments indicative for proteoglycans are markedly reduced in size with advancing age in centrally located regions of costal cartilage. Treatment with testicular hyaluronidase previous to acridine-orange staining leaves these small proteoglycan filaments unaffected. By contrast, the filaments visible after acridine-orange staining in the extracellular matrix near to the perichondrium are susceptible to hyaluronidase treatment. Infrequently, a sharp increase in collagen fibril diameter can be observed in territorial matrix areas of degenerating chondrocytes. This observation is conspicuous at ages of 10 and 20 years. Amianthoid transformation is characterized by the appearance of collagen fibrils strictly arranged in parallel. These amianthoid fibers are embedded in a matrix rich in small acridine-orange-positive filaments similar to the proteoglycan filaments observed in centrally located matrix regions. It can be concluded that extensive remodelling not only of the collagen fibrils but also of the cartilage proteoglycans is involved in the development of amianthoid transformation.