The objective of the present clinical study was to evaluate the predictability of a treatment procedure (GTR) aimed at regenerating periodontal tissue. 34 patients and 103 teeth were selected for the investigation. All teeth included in the study exhibited advanced loss of periodontal attachment. The periodontal defects were associated with even and/or angular bone defects or, in the case of multirooted teeth, advanced loss of periodontal tissue support in the furcation area. The probing bone level (PBL) and the probing tissue level (PTL) of the diseased sites were recorded using a standardized procedure. Treatment included placement of a barrier membrane (e-PTFE, Gore-Tex) according to the GTR method. After a healing period of 4-6 weeks the membrane was removed in a second surgical procedure. The result of healing was evaluated immediately after membrane removal. A further measurement was carried out during a re-entry operation 9 months later. The measurements of the treated sites revealed that GTR therapy had resulted in a marked tissue gain of more than 60% (p less than 0.001). In 33 of a total of 34 furcation-involved molars treatment had resulted in complete closure of the furcation defect. In the light of the findings of the present clinical trial it may be suggested that GTR therapy is an effective and predictable means of improving prognosis for both single- and multirooted teeth. A definite evaluation, of course, will be possible with the results of the re-entry procedure.