THE PRESENT STUDY was undertaken to verify whether acid conditioning of the root surface can promote new connective tissue attachment in man. A flap was raised over two maxillary central incisors, with 6-mm pockets, which had to be extracted in a 72-year-old woman. Following root planning, a rectangular area was delineated by a groove on the buccal surface of the teeth. Inside these limits, cementum was removed and the denuded dentin was etched with a 50% gelified phosphoric acid solution (pH, 1.3) for 1 minute and the flap was repositioned. Forty-two days later, the experimental teeth were extracted, and undecalcified specimens were prepared routinely for transmission electron microscopy. Results showed that new connective tissue attachment had occurred between healing connective tissues and acid-treated dentin surface, with functionally oriented Sharpey's fibers. The mechanism of new connective tissue attachment could be observed as a typical cementum crystallites deposition on a denuded dentin matrix coupled with an active collagen fibrils synthesis. This process resulted in the incorporation of fibers into the new superficial layer of cementoïd tissue.