Objective: To evaluate the efficacy of ß-tricalcium phosphate (ß-TCP) (Synthograft, Bicon USA) in periodontal osseous defects in comparison to open flap debridement (OFD). Method and Materials: Twelve patients showing clinical and radiographic evidence of almost identical bilateral infrabony defects were selected. The infrabony defects in the 12 patients were treated with OFD+ß-TCP on one side and OFD on the other side. Clinical evaluation was performed at baseline and 6 months following therapy. Results: No differences in probing depth (PD) reduction or clinical attachment level (CAL) gain were observed, although a statistically significant difference was observed for the defect fill between the two groups. Six months after therapy, sites treated with OFD+ß-TCP showed a reduction in mean PD from 9.67 ± 2.35 mm to 4.00 ± 1.60 mm (P .05), a change in mean CAL from 9.92 ± 3.15 mm to 5.00 ± 3.86 mm (P .05), and the mean defect fill was 2.92 ± 0.90 mm. In the sites treated with only OFD, the mean PD was reduced from 7.58 ± 1.08 mm to 2.67 ± 0.65 mm (P .05), the mean CAL changed from 6.83 ± 1.34 mm to 1.83 ± 1.64 mm (P .05), and the mean defect fill was 0.83 ± 0.39 mm. Reduction of 5 mm in PD was observed in 5 of the 12 defects (42%); 4 of the 12 defects (33%) gained 3 mm of CAL in the test sites. PD reductions and CAL gains of 3 to 6 mm were measured in the majority of the cases (60% to 75%) regardless of treatment modality. Conclusion: Within the constraints of this study, both therapies resulted in significant PD reductions and CAL gains 6 months after surgery. Sites treated with OFD+ß-TCP showed a significant defect fill compared to those treated with OFD alone. Schlagwörter: alloplasts, ß-tricalcium phosphate, bioceramics, bone graft, infrabony defects, regeneration