The goal of periodontal therapy includes not only the halting of the progressive loss of connective tissue attachment but also the restitution of those parts of the supporting apparatus which have been destroyed. The cells which repopulate this area may originate from any number of the different tissue components of the periodontium: the dentogingival epithelium, the gingival connective tissue, the alveolar bone, and the periodontal ligament (PDL). The size and shape of the wound and the potential of the different types of tissues cells to migrate, determines the type of healing. Previous studies have revealed that root resorption was found to occur when granulation tissue derived from the gingival connective tissue or alveolar bone was proliferating into contact with the root surface, while re-epithelialization of the surface prevented both connective tissue attachment and root resorption. The only cells which appear to have the capacity to form a new attachment were cells originating from the PDL. Based on this knowledge of the biological behaviour of the different periodontal tissues during wound healing, the principle of Guided Tissue Regeneration (GTR) was introduced and tested. A membrane section was placed over the debrided root surface in such a way that all periodontal tissues except the PDL cells were prevented from reaching contact with the root. Healing resulted in extensive amounts of new connective tissue attachment, that is, the formation of new cementum with inserting collagen fibres. The objective was, and is, assessment of how predictably new attachment may form when treatment included GTR.(ABSTRACT TRUNCATED AT 250 WORDS)