The purpose of the present study was to evaluate the effectiveness of granular hydroxyapatite as an alloplastic bone implant material in infrabony periodontal defects. Thirty-five infrabony defects in 28 subjects with marginal periodontitis were used. The patients were instructed in oral hygiene and received several sessions of scaling and root planing. Granular hydroxyapatite was implanted into test sites with infrabony defects following application of internally beveled full thickness flaps, root planing, and defect debridement. Pocket depth and attachment level were recorded before and 3 and 6 months after surgery. Gingival recession, mobility of the tooth, plaque index, and gingival index were also recorded before and 1 and 2 weeks, and 1, 3 and 6 months after surgery. Redness and swelling of the gingiva and increase in tooth mobility appeared transiently after surgery, however, they disappeared in a short time. A significant reduction of mean pocket depth of 3.4 mm and a significant increase in the mean attachment level of 2.5 mm were observed at 6 months after surgery. The mean gingival recession during the experimental period was only 0.9 mm. The amount of hydroxyapatite remaining implanted in the infrabony defect was 73% 6 months after surgery. The data and clinical impression suggest that hydroxyapatite has a potential as an alloplastic implant with clinically apparent acceptance by soft and hard tissue.