Histologic studies of periodontal reconstructive therapies have repeatedly shown a zone of nondescript connective tissue adaption, or collagen adhesion, to the root surface between the apical extension of the junctional epithelium and the coronal extension of identifiable regenerated cementum. To clarify the nature of this zone, supra-alveolar periodontal defects were created bilaterally in the mandibular premolar region in three beagel dogs,and exposed roots were instrumented to remove the cementum. Flaps were then coronaly advanced and sutured. Histologic analysis after 12 weeks of healing showed instrumented root dentin not associated with junctional epithelium, new cementum, root resorption, or ankylosis in 27 of 60 root surfaces. In 10 of these specimens subsequently examined by transmission electron microscopy, collagen fibrill were generally oriented parallel to and in close proximity to the root. A mechanism of collagen attachment was, in fact, functioning at sites of collagen adhesion; these areas may validly be included in the histologic attachment level reported in studies of periodontal reconstructive therapy.