The present clinical trial was designed to evaluate the regenerative potential of the periodontal tissues in degree II furcation defects at mandibular molars using a surgical treatment technique based on the principles of guided tissue regeneration. The patient sample included 21 subjects, 22-65 years of age. The patients selected had periodontal lesions in the right and left molar regions including advanced periodontal tissue destruction within the interradicular area. After an initial examination, each patient received a series of full-mouth scalings and root planings. 2-3 months later, they were recalled for a baseline examination including assessment of plaque, gingivitis, probing depths and probing attachment levels. The furcation involved molars were randomly assigned in each patient to either a test or a control treatment procedure. The test procedure included the elevation of mucoperiosteal flaps at the buccal and lingual aspect of the alveolar process. The inner surface of each flap was carefully curetted to remove epithelium and granulation tissue. The root surfaces were scaled and planed. A teflon membrane was adjusted to cover the entrance of the furcation area and the adjacent root surfaces as well as a portion of the alveolar bone apical to the crest. The flaps were repositioned and placed on the outer surface of the membrane and secured with interdental sutures which were removed after 10 days of healing. Following surgery, the patients were instructed to rinse the mouth twice daily with chlorhexidine gluconate. A second surgical procedure was performed after a healing period of 1-2 months to remove the teflon membrane.(ABSTRACT TRUNCATED AT 250 WORDS)