Objective: The aim of this study was to evaluate the effect of guided tissue regeneration (GTR) alone and in conjunction with a bovine inorganic bone matrix in furcation defects. Material and methods: Twenty class III furcation defects were treated in 18 nonsmoker patients, 35-75 years old. Horizontal (CAL-H) and vertical clinical attachment levels (CAL-V), probing depths (PD), gingival margin levels (GML), horizontal (BDL-H) and vertical bone defect levels (BDL-V), and alveolar crest levels (ACL) were performed at baseline and at 6-month re-entry procedures. Subtraction radiography was used to assess gain or loss in optical density (OD) and area of bone fill (A) (baseline/6 months). After flap elevation, the sites were randomly assigned to receive GTR + Bio-Oss((R)) (test) or GTR treatment alone (control). Results were evaluated using ANOVA. Results: Differences were statistically significant between baseline and re-entry for PD, ACL (p < 0.01) and GML (p < 0.05) for the control group, and for BDL-V (p < 0.01) for the test group. There was a gain in ACL for the test group and a reduction in ACL for the control group (p < 0.0 1). No differences were observed for OD and A. Conclusion: The results of this study indicate that class III furcation defects are not predictably resolved utilizing GTR or GTR in combination with an inorganic bone matrix.30118