The purpose of the present study was to compare the clinical outcomes of infrabony periodontal defects following treatment with an anorganic bovine-derived hydroxyapatite matrix/cell-binding peptide (ABM/P-15) flow to open flap debridement. Twenty-six patients, each displaying one infrabony defect with probing depth>or=6 mm and vertical radiographic bone loss>or=3 mm participated in the present study. Patients were allocated randomly to be treated with ABM/P-15 flow (test group) or open flap debridement (control group). At baseline and at 12 months after surgery, the following clinical parameters were recorded by a blinded examiner: plaque index, gingival index, probing depth (PD), clinical attachment level (CAL), and gingival recession. Both treatments resulted in significant improvements between baseline and 12 months, in terms of PD reduction and CAL gain (p<0.001). At 12 months following therapy, the test group showed a reduction in mean PD from 7.8+/-1.6 mm to 3.5+/-1.0 mm and a change in mean CAL from 8.5+/-2.1 mm to 4.6+/-1.2 mm, whereas in the control group the mean PD decreased from 7.5+/-0.8 mm to 4.9+/-0.7 mm and mean CAL from 8.2+/-1.2 mm to 6.4+/-1.4 mm. The test group demonstrated significantly greater PD reductions (p=0.002) and CAL gains (p=0.001) compared to the control group. In conclusion, treatment of infrabony periodontal defects with ABM/P-15 flow significantly improved clinical outcomes compared to open flap debridement.