The aim of this study was to evaluate the predictability of guided tissue regeneration (GTR), using ePTFE-membranes (Gore-Tex) in the treatment of advanced periodontal disease. The study presents long-term results for 88 teeth in 23 patients at least 9 months after membrane surgery. The periodontal lesions included severe horizontal and/or vertical bone loss. The bone level (BL and BL') and the tissue level (TL), a new parameter between cemento-enamel junction and coronal margin of the tissue in the defect, were recorded during surgery: immediately before application of the membrane (BL), after membrane removal (TL) and during a re-entry procedure (BL') 9 to 12 months later. The average tissue gain in the periodontal defect (BL-Tl) at membrane removal was 65.7% (p < 0.001) and the average gain in mineralized tissue at re-entry (BL-BL'), 30.4%, meaning more than 46% of the gained tissue at removal was mineralized at re-entry. The decreased amount of mineralized tissue at re-entry in relation to the tissue gain at membrane removal might be due to formation of a so-called long connective tissue attachment or to mineralization-induced shrinkage of the new tissue and some surgical difficulties in coverage of the newly formed tissue. Nevertheless, an absolute gain of 31% mineralized tissue after GTR can provide a marked improvement in the prognosis of a periodontally severely damaged tooth.