Following the examination of diseased and experimentally damaged blood vessels with the electron microscope, it has been possible to describe two fundamental reaction patterns of the vascular collagen. From the point of view of their ultrastructural appearance, these may be designated intrafibrillar and interfibrillar collagen dysplasia. The term intrafibrillar dysplasia includes all ultrastructural anomalies of the single fibril. The term interfibrillar dysplasia is reserved for changes of the entire bundle of fibrils into a form no longer functionally adapted to the demands made on the vessel wall.