The authors report the clinical and histologic data on the healing of a severe periodontal lesion obtained in a one-walled intrabony defect using anorganic bovine bone under a bovine peritoneum-derived collagen membrane. Eight months after surgery, a bone-like tissue replaced the lost tissues. A biopsy of this tissue was carried out. In the part of the specimen closer to the residual bony wall of the original defect, anorganic bone particles (ABP) appeared to be surrounded by a layer of newly formed bone; its osteocyte lacunae were colonized by osteocytes from the host, and actively secreting osteoblasts were observed in many microscopic fields. No resorption phenomena were observed in the ABP. Newly formed cementum with actively secreting cementoblasts was present on the tooth surface, and well-oriented fibers inserting in both newly formed cementum and bone were observed. In an area far from residual bone, all ABP did not appear to be surrounded by newly formed bone. Osteocytic lacunae appeared not to be colonized by cells, and ABP was surrounded by dense connective tissue without osteoblasts near the grafted particles. A very limited amount of newly formed bone, without relations with ABP, was observed close to the root surface. From a clinical point of view, anorganic bone in association with a collagen membrane can be effective in the treatment of bony defects characterized by an unfavorable architecture. From a histologic point of view, the clinical appearance of bone regeneration is not always confirmed in the part of the defect far from the bony walls.