Infrabony defects of 230 periodontal patients have been treated surgically by implantation. These lesions of alveolar bone, up to nineteen per patient, generally displayed pocket depths of at least 8mm and were associated with teeth showing pronounced mobility. Approximately 50% of these teeth could have ordinarily been condemned to extraction. In this study, scaling, root planing, and polishing were performed and patients were instructed in oral hygiene measures prior to surgical implantation. A 1:1 mixture of dense sintered hydroxylapatite(HA) particles (OrthoMatrix HA-500) with plaster of Paris (USG Medical Grade Calcium Sulfate Hemihydrate B, containing 0.85% K2SO4) was moistened with a minimum amount of saline. The mixture was then implanted into the dried defect. After setting, the operative site was closed with silk sutures. The plaster bound the HA particles in the defects and prevented particle straying which frequently occurs when they are implanted with saline alone or with other vehicles. There was an almost immediate and sustained diminution in tooth mobility at most implantation sites. Good retention of particles was seen on physical and radiographic examination of most defects for the length of the study (5 years). Overall, there was a marked reduction in pocket depth and improvement in tissue tone and texture. Less than 1% of the teeth at the implantation sites have been lost. No significant inflammation has been observed. Increased radiopacity and resistance to probing suggest bone formation in some implanted defects. Implantation of periodontal osseous defects with the biocompatible HA/plaster composite appears to result in the clinical cessation of progressive periodontal disease.