The present investigation was undertaken to quantitate the osseous changes which occur throughout the entire circumferential extent of infrabony periodontal defects in patients with optimal plaque control. Fifteen defects were selected in nine patients. Periodontal surgery was scheduled after each patient had shown an ability to practice efficient plaque removal. Muco-periosteal flaps were raised and the osseous defects debrided. The dimensions of each defect were measured at several specific location points within the defect. The flaps were replaced at their original location and, post-operatively, optimal plaque control was maintained in the area (Mean Plaque Index 0.04). Six to 8 months after the initial surgery all areas were re-operated and the osseous defects were remeasured at the same specific location points. Each defect showed osseous regeneration at every location point. The mean initial osseous defect depth at a location point was 3.5 mm and the mean amount of bone regeneration which occurred was 2.5 mm. Crestal alveolar bone resorption occurred at almost half of the location points and averaged 0.7 mm. Eleven of the 15 defects had resolved completely. There were isolated areas where a shallow defect persisted in the remaining four defects. The behavior of an osseous defect throughout its circumferential extent was characterized by a combination of coronal bone regeneration (mean 77%) and marginal bone resorption (mean 18%). Infrabony periodontal defects may predictably remodel after surgical debridement and establishment of optimal plaque control.