Twenty-three human subjects with two Class II furcation involvements in lower molars were treated with initial therapy following which presurgical measurements of pocket depth, gingival recession and attachment level were made. Periodontal flaps were used to expose the furcation defects, and one defect was implanted with porous hydroxylapatite while the other served as an unimplanted control. At the time of surgery, bone defects were measured obliquely and horizontally using a specially designed device to ensure reproducible probe angulation. Six months later the presurgical measurements were repeated, and reentry surgical procedures were carried out to measure the changes in the bone defects. Areas implanted with porous hydroxylapatite showed a statistically significant reduction in pocket depth and a statistically significant improvement in attachment level and fill of bone defects when compared with control defects. There was statistically less gingival recession in the implanted areas compared with the control sites. Control sites at six months showed no significant change in pocket depth, an increased loss of attachment and worsening of the bone defects.